The problem I see is that in a long fight you could easily run out of all your encounter powers and be stuck using only at-wills to finish the fight. At that point, for many characters, there are no more actual decisions to be made - which IMO makes the rest of the fight pointless. If your actions are predetermined, you're not even playing a game, you're just killing time.
Now it's been pointed out that several classes in 3E had this problem much more than any 4E class does. However, to me, and probably a fair number of people, this was irrelevant, because I didn't play characters without options (and spellcasters are not the only way to have options).
It's like a buffet where half the stuff is incredibly delicious, and the other half is flavorless survival rations. Then they replace it with a buffet where everything is average quality, fairly tasty but nothing special. If for some reason you were eating the survival rations before, this would seem like an improvement. But if you were eating the good stuff before, it's a downgrade.
Now before anybody jumps on this, I'm not even talking 4E in general, just the relevant number of options in combat specifically.