It wasn't playtested by WotC as extensively as Heroic, so a lot of effects are incredibly annoying and lock-down-y. Both by PCs and monsters. I ran a group of 30th level PCs against Level 35 solo Lolth, AND I beefed her up, and they still completely owned her. Admittedly that's prominently due to the poor design of solos.
Oh, he tripped you? Well, since you're equivalent to five monsters, he basically stole 5 move actions.
Oh, he's a revenant swordmage who teleports you away from his allies whenever you try to attack him, and who's insubstantial and nigh unkillable when he's at negative HP. Oh, did I say nigh unkillable? I meant literally unkillable, because the legendary warlord simply won't let him die while he's up, and the swordmage won't let you attack that warlord.
Or consider mind flayers, which stun (save ends) with an at-will. Or bards who can impose a -7 penalty to a monster's attack roll, again as an at-will.
Whereas 3e had lots of one-hit kills (and countermeasures you had to put in place for them), 4e has lots of 'you'll-never-win' effects, and practically no counter-measures.
