Hmm, perhaps the angry peasant mob is made up of hard-core 1E fans upset that their cover art has been changed?
Seriously, when I read the title of this post I thought, "Oh boy, here comes another one..."
Thank God this is an in-game post!!
Well, in that case, how about a daily power called "You Changed My Cover Art!" as an immediate reaction?
In real terms... interesting question... I'm not sure quite how to express it in 4E mechanics without thinking on it more, but some sort of flexible "attack scale" might be interesting -- at full strength, the mob can do substantial damage, but as it "takes damage" (which is to say, as the peasants comprising the mob get killed), its attack abilities decline somehow. I'll have to think on that some more for how to express it best in 4E...
Actually, that was something that I was considering as well. I think that I'll put a lot of the nasty damage and powers up front. Once they get below bloodied, the mob will start to disperse. A few may stay...but most of them will leave.
Anyway, here are a few items that are on the table for consideration:
vulnerable to area effects
push, pull, and grab powers
split ability (I'm considering this one...it was one of the suggestions from the Enworld thread).
"It's Only a Kid!" - immediate interrupt, recharge, vs Will, turns a hit into a miss
I'm also thieving some powers from the kobold example in Dragon magazine (also mentioned and linked in the Enworld thread).