Skrolk became marginally better with the 8e rules. The first thing that stood out was his Liber Bubonicus. In earlier editions, rolling a 2 - 4 was basically useless against T4 units. However, with the change to the To Wound chart, even the lowly S1 hit can take out some of the more resilient units in the game.
Since Skrolk causes Terror (and Fear), his aura of pestilence becomes exceptionally useful when the enemy fails his fear test, meaning the enemy will be hitting Skrolk on 6's.
But by far, one of the most useful things about Skrolk is that he makes Plague Monks count as Core choices. Now, I'm not suggesting that you fill up an army with Plague Monks (although that is a terrifying thought), but I think its benefit lies in freeing up your Specials points to be spent elsewhere. Those points could easily go for more Plague Censure Bearers, or another pack of Rat Ogres.
Thanquol & Boneripper:
Thanquol ranks among the best wizards in the game. With the new magic phase, warpstone tokens are excellent for phases in which you find yourself lacking power dice. And, since Thanquol gets D6 +2 tokens means that you have plenty of reserve power at your disposal.
Boneripper is somewhat of an afterthought. A lone rat ogre, albeit a heavily fortified rat ogre, isn't much of a punch on the battlefield. His warpfire thrower will help if he gets charged, and since he's unbreakable, he'll at least hold up a unit for a few turns. I tend to use him to cover a flank if I can.
Now, while I don't plan on covering entirety of the Skaven spells, I think special mention needs to be made for Curse of the Horned Rat. This spell isn't going to be quite as useful as it used to be, simply because 8e favors larger units, and to get the best out of CotHR (i.e. clanrats), the target unit should have 14 models or less. Also, the insanely high casting requirement of 25+ means that its going to be very hard to cast without IF/miscasting, and uses up quite a lot of power dice in the process.
Don't get me wrong, its still a brutal spell to use against your opponents elite infantry units, causing 4D6 casualties with absolutely no saves is always fun. And just to prove how useful it can be, keep in mind that Slann are infantry models, so a lone slann, or one in a suitably small unit, can be instant killed with CotHR.
Ikit has my favorite fluff of all the Skaven, so I'm a little disappointed in his rules. That aside however, he'll do very well leading a unit where his meager 2 attacks won't get him killed.
The only problem I can see is Ikit's Storm Daemon. With the new bound spell rules, a miscast will mean that he'll be without his weapon for the remainder of the game.
However, since the power dice pool is back to being communal for wizards, rolling a 6 when it misfires will be very useful.
And, as with boneripper, Ikit's warpfire thrower will help when he gets charged, especially since he gets to reroll the artillery die, hopefully letting him be more accurate.
Throt the Unclean:
Throt wasn't really affected all too much by the 8e rules. He still adds one hell of a punch to any Moulder unit he joins. Although I still recommend adding as many packmasters as you can muster if you plan on putting him in a unit of Rat Ogres to act as ablative wounds.
Since he causes Fear, I tend to put him into giant rat packs. First, I don't have to spend points on extra packmasters, the giant rats make a tasty meal on the cheap. And since he causes fear, a failed fear test means that all the rats will be hitting on 3+, something that they seldom get to do.
As far as rules changes, it's subtle but worth mentioning. The FAQ cleared up that the whole unit suffers -2 Ld if it suffers a wound from Whip of Domination, so if you win combat, you can pretty much expect to be chasing your opponent down. In addition, it will help against all those pesky Stubborn or steadfast units (unlikely, unless he's with Rat Ogres).
Like Skrolk, Throt's best feature is his ability to make some Rat Ogres count as Core choices, freeing up some Specials slots for.....more Rat Ogres! Having Giant Rats actually count as Core choices is nice, but unless you're playing a Moulder heavy army, it doesn't really make much of a difference.
Warlord Queek Headtaker:
As with Throt, Queeks functionality changed very little in 8e. The biggest boon of queek is that his low points cost means he can fit comfortably into a 1000 pt game, giving you quite a combat monster for a game so small. And skaven being skaven, you'll still most likely outnumber your opponent even with Queek eating up a fifth of your points.
I'm really disappointed in Queek's extra ability, buffing a unit of Stormvermin. While it does boost their combat prowess, giving them S6 attacks, at 4 pts a model, its a bit too pricey for my tastes. Comparatively, Dwarf Longbeards get the same stat boost, PLUS Immunity to Panic, PLUS the ability to allow nearby units to reroll failed panic tests, all for 1 pt less per model. That being said, you're not going to get that many S6 attacks any other way in a Skaven army, so if you've got the points to blow, go for it.
Skaven don't have many quality over quantity units, but the Verminlord is certainly one of them. With 5 attacks, I 10, S 6, and Multiple Wounds (D3), he is a monster slaughterer. And on top of that, he's a Lvl. 4 Wizard who can have Curse of the Horned Rat. I'm hard pressed to find a reason not to take him.
The only reason I can find is that he's 500 pts, so unless you're playing a truely epic sized game, its either the Verminlord or a Screaming Bell.
As a spellcaster, I have mixed feelings. On the one hand, he's not as versatile as a Grey Seer due to lack of warpstone tokens, and his inability to join units. On the other hand, he's one of the few wizards in the game that can survive a miscast or two without crippling himself, and since he fights alone, miscasts won't often harm your other troops.
Warlords and Chieftains:
These generic characters bear almost no mention, as they have no special rules that would be affected by 8e rules.
For the most part, I'd never take a warlord, opting for one of the named lords, or a verminlord or grey seer. However, if mounted on a rat ogre bonebreaker, a warlord might just merit a spot in my force. The bonebreaker is like a rat ogre on steroids, which adds considerable clout to the skaven warlord during a challange. But the biggest reason to bring one is that he causes Fear, so dropping one into a unit of clanrats or stormvermin will give you effectively an extra fear causing unit.
Chieftains have one redeeming quality. They're cheap. They're perfect for adding to units to boost their Ld., and can make the best use out of verminous valor, so they dont' get slaughtered in challanges.
Grey Seers are great wizards, with a little versatility since they get some warpstone tokens to start, and can buy more as you see fit. Their ability to take Skitterleap as a default spell is fantastic, allowing you to position your characters exactly where you need them.
A particularly favorite tactic of mine is to Skitterleap the grey seer behind enemy lines or along a flank, and then deliver a hopefully devastating Plague, Vermintide, or Cracks Call.
But the real reason to take a grey seer is to bring a Screaming Bell to the party. While its effects may not be completely predictable, all are useful. One of the many nice things about a Screaming Bell is that it takes up so much space in the unit, that making a horde out of the unit pushing the bell is much easier.
Deathmaster Snikch is quite the hero. However, in 8e, its well that he is since he can't be the army general, you don't need to use up valuable Lords points. He's still one of the best ways to take out enemy war machines, but he can still hold his own against enemy characters.
Another great use for Snikch, is to put him in a tunneling unit of Gutter Runners. It (hopefully) will get him into combat quickly where he can reap a large body count.
Tretch is a really solid hero choice, and with the new steadfast rule, he's much easier to use to fullest effect.
Tretch can help cripple those particularly hard to shift units with the help of a few units of clanrats or stormvermin. Basically the idea is to engage the enemy with one unit, stay locked in combat (this is where being steadfast helps), and then when you charge the flank with the second unit, you can use the Stay Here, I'll Get Help rule to hop him over to the flanking unit to allow them to reroll To Hit rolls.
Okay, although it has nothing to do with the 8e rules, I do need to vent my frustrations about skaven assassins. After the newest edition of the Dark Elves Armies book, I was hopeful that the Skaven assassins would get the same treatment. Alas, this is not the case, so we're stuck shelling out valuable heroes points for assassins. It's frustrating to say the least.
Now that I've gotten that off my chest, its fair to say that skaven assasins are basically less potent versions of Snikch. And for the points, you can afford two lightly armed assassins for the same price as the Deathmaster. So really, as far as assassins go, it comes down to if you want one big slaughtermaster, or two smaller slaughtermasters.
Of course, you could add more assassins if you have more points to spend, but I tend to not go hog wild with the assassins, since there are so many other great things to take.
Warlock Engineers are an interesting choice that isn't at first apparent, but with a 15 pt price tag, the ideas are fast coming.
I like to think of them as Brass Orb delivery systems. Just skitterleap one of these guys into range of a low Initiative unit and hurl the globe. With a little luck you'll cause some massive damage, and if the little guy ends up slaughtered by some disgruntled enemy units, you've only really lost 15 pts. The same tactic can be employed with a Death Globe, but can be equally deadly against any unit.
Alternatively, I occasionally charge these guys with leading my slaves into battle. I'm willing to spend 15 pts to give my slaves +3 Ld. It comes in handy when the general isn't around. (The lifespans of my warlock engineers are very short.)
However, with the new rules, they're really handy when upgraded to wizards. I generally only upgrade them to Lvl. 1 Wizards, because the whole point is that they cost so little. At 65 pts, only goblin shamans are cheaper, and they allow you to channel extra power dice.
Plague Priests are a wonderful add-on to any Pestilens unit. The Ld 6 helps keep the Frenzied troops from charging unnecessarily. Since he's a Lvl. 1 Wizard, he can act similarly to the Warlock Engineers channeling power dice.
On top of all that, he's got a pretty epic stat line, boasting T5, and S4. With a flail, he's making a whopping 4 S6 attacks in the first round of combat. Or, with a Plague Censure, he's even at home in a unit of Censure Bearers.
However, the biggest benefit to the Plague Priest is to bring the dreaded Plague Furnace to battle. Not only does it give you an Unbreakable unit of Plague Monks, and take up a lot of space, allowing you to field a horde of Monks relatively easily, but adds lot of punch to combat with the possibility of 10 S5 auto hits in addition to D6 Toughness tests. In all honesty, I'd be incredibly surprised if any unit survives a charge from a Plague Furnace.