My take on tiers
We have three players in our house and 7 armies to play, started doing partial simulations and play-testing in June when book available to look at in stores and have people in area reporting on games in 8th edition. Here's our view:
Tier 1+: Skaven
Tier 1: Warriors of Chaos, Daemons
Tier 1-: Dark Elves, Lizardmen
Tier 2+: Empire, Tomb Kings(?)
Tier 2: High Elves, Dwarves, Beasts of Chaos
Tier 2-: Vampire Counts, Orcs and Goblins, Ogre Kingdoms,
Tier 3: Bretonnians, Wood Elves
It is too early to tell but our play-testing suggests that Daemons of Chaos no longer have the big advantage that they once had but they have a strategy with Heralds of Tzeentch (loremaster option) and Lore of Life and Bloodletter hordes and blocks with Heralds of Khorne and Lord of Change or Bloodthirster that will make them very tough. Plaguebearers got seriously nerfed, but daemonettes and steeds are improved. Flamers, bloodcrushers, Beast of Nurgle and Fiends all got better. Thus, when all shakes out, Daemons will still be tier 1 but not tier 1+ like before but with less horror units, less plaguebearer untits, and more bloodletter and daemonette units in core.
High Elves were helped on one hand (effectively a form a hatred for high I plus ASF and ASF unit with great weapons) but hurt by the high cost per model, their low S and T, and the step up rule allowing for attacks back. Maybe an archmage with Book of Hoeth will work with a cheaper lord. They are improved slightly, but not tier 1, tier 2.
Skaven were beginning to figure out what works and dominate the local Indy GT's and will only get better. They benefit from: more rare choices and ability to use more of them with the percentage rules; no guess range (warp lightning cannons); multiple lords and cheap heroes to boost LD of units; BSB rules for lord on bell; horde rules; steadfast rules; flyers having less charge range and infantry having greater effective average charge ranges; swiftstride confers less advantage in pursuit and flee than old 3D6 for faster units with M7+; no distruption of ranks unless hit in flanks by a ranked unit; wound on 6's regardless of T and S; step up rule; and okay to decent initiative. Skaven will likely be tier 1 if not 1+.
Warriors of Chaos look really tough in 8th edition. High initiative, T, S, AS and Marks all became more valuable and make them very tough in combat; armies must choose lores with army, so less metal is likely to show up; access of ogres (which are nasty in 2 ranks of 3) and dragon ogres look good, as with shaggoth; warshrine is good for the boost; Lvl 4 sorcs with MoT still allow for cheap lord (boost LD); Tzeentch lore is really excellent in 8th edition with flickering fire, call to glory, pandemonium (RIP, enemy characters cannot give LD to units and miscasts on doubles), treason (only not good against daemons and undead), and gateway all effective and easily cast with lvl 4 MoT giving a +5 casting boost; puppet and will (DP) are really good; have fast cav with vanguard rule and axes now quick to strike; chosen now more viable; only chaos knights and warhounds (don't screen like the past due to TLOS) are less effective than before. Tier 1 without a doubt.
I expect lizardmen to stay between tiers 1 and 2 but the Slaan with extra PD per spell and Lore of Life could change that to tier 1: low initiative hurts but high S, T, and AS is boosted and kroxes get stomp; Slaan gets those extra power dice per spell which is a big advantage over other armies; can't run two EOTGs hurts but can run oldblood cheap and scar vets more often; terradons got hurt but Sallies and Razors as good as ever; skinks have quicktostrike with javelins but not blowpipes(?? cant find in FAQ) which makes them viable; cold one LD tests are helpful.
Dark Elves were definitely hurt in parts but are likely to remain competitive with tier 1, just not one of the three best as before: +can now run two hydras and 2 RBTs or one hydra and 4 RBTs at new standard of 2400 to 2500; spearmen are better; +hydras get to use breath weapon once but in combat, beastmasters get 6 attacks and protected by hydra, hydra gets thunderstomp autohits but hydras have lower I; +dark riders have vanguard rule but shorter average charge range; +corsairs with frenzy banner and Repeater hand bows (quicktostrike) or AHWs got better if run with Cauldron; the cauldron is slightly less protected but also more effective due to ward save, +1 attack and killing blow blessing; +can now run a Lvl 4 Sorc and a dreadlord (cheap); +can now run one more hero; +lots of protective magic items and means of generating extra power dice above winds of magic; ++RXB warriors are substantially improved with shooting in ranks and musicians allowing a free reform and the shield giving +1 AS and 6+ parry save; witch elves lost a lot; execuitioners are not longer playable; harpies are far less effetive due to low LD and having to LD test to march if close to enemy; terror tests and fear tests are now less threatening and were a serious problem for DE units; the 25% core rule is less harmful to DE armies given that they have four viable core choices with 3 of the 4 core units improved in 8th edition relative to 7th edition. DE, thus, will not be as strong as before relative to certain other armies but will likely remain tier 1 or 1-.
Empire I don't know yet. Stanks may be better than before with T10 but also can be killed by certain spells; cannons are definitely improved; Steadfast helps but troops and shooting is not as impressive in comparison with other armies in terms of I, WS, T, and S; and detachments may be good for disrupting ranks. But the cavalry are less effective and new bound spell rules (must use PD to cast) will cut down the effectiveness of war priests. It may take a while for empire armies to figure out what works and they will be as or more competitive but maybe somewhere between tier 1 and 2 or stay at tier 2.
Vampire counts are seriously nerfed and drop from tier 1 to tier 2-. -Winds of magic often limits and makes less reliable the number of power dice in each turn and limits the difference between power dice and dispel dice unless certain vampire items are taken. -The ability to cast a lot with single dice is less likely (unless last spell attempt) because of roll of a 1 or 2 ends the ability of the caster to cast. -The base caster levels are only 1 and 2, not 4, and that puts VC casters at a disadvantage in casting and dispelling relative to certain other armies. Bound spells must now use power dice to cast. -The most competitive vampire armies had heavy lord cost and hero costs and heavy focus on special and rare points and min core units., while skellies and ghouls are playable they are not cheap relative to stats (low initiative) and will eat up points spent in the past on other units and characters. -Fear and terror are not as good as before. -Supporting attacks in ranks means that zombie roadblocks can be wiped out more easily. -No shooting (which is more important in 8th edition). -Less ability than some other armies to go after war machines with flying and fast cav.
Don't know yet about dwarves, They've improved with steadfast, step up, war machine shooting no longer have guess range, and shooting. They move from the bottom tier to maybe tier 2- but just don't have enough movement and magic (better magic defense).
O&G gets boosts from horde rule, steadfast, casting and dispel dice special rules, ability to spam cheap lords and commom magic items. They move up to tier 2- from one of the worst, if not the worst army. Animosity and bad BS shooting really hurts them but doom diver and some other shooting may be a bit better. Rumored new book next spring might get them up further.
Beastman look better and may go from one of worst to tier 2. Ogres benefit a lot but also go from one of worst armies to tier 2- and will still struggle until a new army book comes out. Change is rank size for monstrous infantry, supporting attack rules, and power dice limits from winds of magic helps both these armies become more competitive.
Brets and wood elves got nerfed. GW should have created special FAQs for Brets to allow cav to have a better chance to disrupt ranks and stuff. Wood Elves need a new book and better lore or to go to lore of life; not enough forests in the terrain in 8th edition to help them.
TK is a lot better than people think when played well. TK has been very competitive in our recent Indy GT's in 7th ed. TK priests now get magic levels 1 and 3 for casters and still autocast with incantations (FAQ 1.1). Also, they will get the first book. They are tier 2 but may compete with tier 1 armies with those catapults (no guess range), chariots, and incantations not using power dice.
This is, of course, based on what people have come up with as 8th edition test lists and possible army composition changes. I used to have 5 tiers and armies in tiers 4 and 5 had almost no chance against tier 1 and tier 2 armies unless the top armies were non-optimal and the tier 1 and 2 armies were toned down (which often happened in our tourneys). I collapsed the tiers to 3 but put + and - ranks to reflect the tighter balance across armies that appears to have occured. The one good thing is that a tier 2 or 2- army now has a decent chance, with some luck or better play, to beat a tier 1 or 1+ army. No army is as dominant as before, at least until someone figures out a broken combo. The percentage allocations/limits and winds of magic rolls balance the game a lot. This is just an educated guess for now.