This is also true for saving throws (nka 'Defenses'). In 3E, especially at higher levels, your good saving throw starts to get closer to 'automatic' (barring rolling a 1), and your other saves you generally are going to fail. 4E evens this out quite a bit, so the variance in saves is no where nearly as dramatic, and you have a decent chance to succeed on any defense.
Eh. There aren't many "only succeed on a 20, only fail on a 1" situations anymore, but with the way the stat increases work, PCs tend to end up with a pretty big variance on defenses. Attacking a Wizard's Fort, for example, is usually a pretty safe bet. Ditto with a Barb's Reflex. And a DM should probably think twice about attacking a Paladin's AC, even with Divine Challenge factored in. Though it is better than it used to be, I suppose.
My personal favorite changes:
-Action economy. No more summoners getting four or five the turns of anyone else and bogging down the game.
-Easy to DM for. Encounter planning is a snap, and the clear use of keywords mean less head-scratching and rule-searching at the table.
-Character creation is super-easy (at least compared to high-level 3.5, it's not quite at Mouse Guard's level yet). Every character is able to contribute to party success in and out of combat.
-Roles system means PCs have clear strengths and weaknesses. This encourages them to work as a team, instead of acting like spellcaster meatshields. Additionally, means that class selection is open to just about anything (no more cleric required since they're the only one who can Raise Dead or remove negative levels). An entirely non-magical party is not only possible, but balanced and effective!
-Magic items are so incredibly simple. Characters only need three to be up-to-par, and get them automatically in creation.
-Having no iterative attacks speeds up the round, and encourages mobility, rather than just standing still and full-attacking.
-No dead levels, as mentioned before. Something new to choose every level.
-Easy, balanced multiclassing. Spellcaster multiclassing is no longer a trap!
That's enough for now, but there's plenty more changes I approve of.