First Defeat, and it Hurt

The game doesn't favour "all comers" lists particularly well - it works better with dedicated tasks, and min-maxed points options. All lists need to cater for all eventualities, but looking at Eldar, whose units, by their very definition, are dedicated to one particular job, this problem is exacerbated and highlighted even more.

Take a look at it this way. Space Marines Tactical Squads are designed to be able to take on any foe the enemies of the Imperium can throw against them - equipped with Bolters, they're extremely effective, and with access to weapons like Lascannons and Missile Launchers, they can take down Monsters and Tanks, and special weapons like Plasma guns lets them be a massive threat even to things like Terminators, or Ork Nobz. That's fine, and perfect from a fluff perspective, similar to how soldiers for today are equipped with rifles, and have access to LMG's, and rockets to deal with harder threats.

However, from a gaming point of view, that "all comers" squad is wasting half of its potential. Without access to split firing, as can happen in real life, that Lascannon is wasted shooting at a horde of hormagaunts while the rest of the squad guns them down, or those bolters just ping harmlessly off the front armour of a land raider.

So, that means that tactical squad's strengths aren't really all that "tactical" or flexible, and can only really be built in one particular format - which takes us back to the days of 3rd and 4th edition, when squads were no larger than 5 men, and took Lascannon and Plasma gun to maximise that option. Now we're "incentivized" to take more marines due to the points/model tax, which channels them into min-maxing a particular squad into anti-infantry purely, which tends to limit them to Heavy Bolter or Missile Launcher (primarily using Frags, although it can, should opportunity arise, use Kraks, but the difficulty of penetrating and causing adequate damage is almost a nonentity), with a Flamer or Plasma. If they're targeting heavy armour they're doing it wrong.

So, on paper, they're flexible, but in the game meta, they're limited to anti-infantry duty, where their S4 AP5 Rapid Fire weapons are somewhat effective. Now, compare that to Eldar, who have units who are dedicated to anti-infantry - either as Dark Reapers, or Warp Spiders with Rending S6 Assault 2 and a Teleport to get close. Eldar do dedicated jobs better than units which are "balanced" to doing multi-task jobs.

If you need armour destroying in a Space Marine army, you have access to several options - usually Devastator Lascannon squads, TriPlas Predators, MeltaDreads in Pods, or Sternguards in squads with Combi-Melta. Compare that to Fire Dragons in a Wave Serpent with Scatter Lasers to ensure they get there. 5 Fire Dragons costs 110pts. For anti infantry, you have Warp Spiders at 190pts, which isn't far off the same cost as the tactical squad - but the Tac Squad is putting out 16 S4 AP5 shots, Flamer and a S4 AP5 Small Blast while the Warp Spiders are churning out 20 S6 Quasi-Rending shots for roughly the same cost.

Yes, there are other mitigating factors, but a combination of speed, low points costs, effective transport vehicles, and psychic powers means that Eldar are better doing dedicated tasks, and Marines can only hope to catch up.

You're still spending 65~ points on models doing nothing.
@Sethis - a very good point - one I meant to expand upon but clearly forgot. Spiders and Serpents are cheap as chips, and maximising their use is one way to get them to work. That clearly leaves one glaring weakness - and that's AV14 Spam, oddly enough, non-Deathwing Assault Dark Angels being one of the better lists to take against Eldar - but that is gotten around by Fire Dragons (who can take more Serpents, Serpents with Lances), and Knights.

Space Marines need a massive shake up - to the extent that Sternguard become pretty much the baseline Marine, but with a large points drop, and Scouts return to their WS4/BS4 self. Las/Plas spam needs to make a return, because currently, what Space Marines bring to the table is all overcosted for their abilities.

I've got to admit that I played more Legion lists than anything else, and Eldar doesn't feature as much in those games, and that Legion lists don't tend to function at a points level any lower than 2K, and arguably 2.5-3K for the most fun, thanks to the points taxes - Praetors/300pts on Boltguns before you can even afford to get anything nice and killy, etc, but what's noticeable is that the Boltgun, and the "much vaunted" resilience of Power Armour is hardly what it used to be. At least back in 3rd edition, Power Armour wasn't counted by anything sans Battle Cannons or 1 shot AT weapons, pretty much. Now, AP3 Blast weapons are a dime a dozen, and due to their medium Strength, lets them work in a light AT fashion, to take out transports like Chimera or Rhino's, and still wipe out swathes of infantry.

So, the strengths of the tactical squad are completely negated by the game. There are 2 stand out options in the Space Marine list - their Flying Potato cheap as chips flyers, and their Artillery.

Flexibility - The game favours specialist weaponry/tasks. Having to choose before game to be AT/AI means that the flexibility is completely gone.
Boltguns - Increased access to cover negates AP5 on weak troops, rate of fire not enough to gun down the hordes.
Toughness - Easy access to high volume low-AP shots/wounds forces units to use cover, which Scouts use better, and as there's no save anyway, you're better using that. Access to Torrent/Template Ignores cover AP3 weapons on high durability units means that even cover is not available. The change to AP for Melee weapons means that at-Initiative AP2 is hard to get, and rarely in any sort of number, which means that loading Power Swords is not as much of an answer as it used to be to deal with them, and assault in general is pretty weak (as a result of that and many more changes, like random charge etc), meaning that shooting takes an even bigger slant, and needs to cover a gap that Banshees used to fill - and high volumes of AP2 fire means that it cuts down the more expensive marines even more quickly.

There's a reason that Drop Podding Alpha Strike Sternguard is one of the most popular builds for Space Marines, and that's because it;

a) reduces the number of shooting phases your Sternguard have to face by 2
b) you get to put powerful AT in the most powerful formation
c) you're forcing your opponent to react to your potential movement - which usually means bubblewrapping units around valuable tanks etc - perfect for allied guard medusa's, tau riptides, eldar Suncannon Knights, and Marine Artillery to put heavy damage on them, or leaving them isolated, and to borrow a term - get them ganked. Conversely it also allows you to place units between intervening terrain to prevent them being slapped silly by the enemy templates.
d) They can be both AT with combiweapons+melta, AI with Bolters/Combi Weapons+flamer/Plasma, or Anti heavy infantry with Combi-Weapons+Plasma - or even at a push, AA, thanks to double Flakk Missiles. Add on that there's a chance of first blood as a result of this, and linebreaker, as well as removing the enemies access to their natural objective should you do well enough, and you're golden.

Vanguard and ASM are kind of poor - without some form of assault from deep strike, they're just expensive marines until that point. Sure, they can kind of move quick, and throw out a ton of attacks, should they all get there - but a canny opponent is just going to negate 150pts worth of models with a single AP3 Large blast.

It's a similar point as well - in Legion lists, in a World Eaters List, I have Assault Marines, or I have Tactical Squads with BP+CCW. Assault Marines have Rage, 1 attack base, and 2 Close Combat Weapons - 4 attacks on the charge, + Hammer of Wrath - a full squad of 20, with no upgrades is putting out 100 S4 attacks. Tactical Squads have the same, but no HoW - so is putting out 80. The difference is about 1 extra dead marine in combat, for a further ~75pts or so IIRC, and yet they die just as easily. A Tactical Squad conversely can have something like a Apothecary added for 5+ FNP, Interceptor, A 2+ armour save on the sergeant, has access to fury of the legion for double shots before an assault, and if you really feel like flashing the cash, have Bolters as well (but that doesn't really fit a WE army, which benefits from having more bodies on the field).

Space Marines suck. They're capable, and they're nowhere nearCSM, Orks, Guard or Nids weak (read Monobuild armies), but they're not capable of swinging with Daemons, Tau, or Eldar.