Two problems, off the top of my head, I have with winging encounters in 4e...
The 1st one is that there aren't any base, run-of-the-mill creatures. I mean if I just want a bunch of Goblin thugs in 3e I flipped to a run of the mill Goblin and only had to deal with one set of stats. Give them all ranged and melee weapons and I'm set. No memorization of unique powers, give them all the same hit points, they all have the same AC, etc. Then if I need a specialized Goblin I used a different stat block or designed it myself.
In 4e I feel like I have to decide if each one (in an off the cuff encounter) is a Goblin Warrior, Goblin Blackblade, Goblin Sharphshooter, and so on. Each of these has at least one or more unique powers which I must keep track of, varying hit points, AC, and so on. Now I also have to consider the makeup of present roles in my party, or this inconsequential encounter could prove way harder or way easier than I anticipated. Now I guess alot of people will say just use minions, but in my experience a fight of just minions is rarely exciting or even worth it, I like them as back up but not as center pieces.