One very important aspect of the mob is this: No one person in the mob can control what the mob is going to do, yet every person who participates shares responsibility for what the mob does. This is why I find flash mobs a scary idea: to volunteer to be part of a mob?
So why go to a flash mob? There is an appeal there - no doubt about it. The mob mentality. Participants are freed from making decisions, and thus feel like they're absolved from individual responsibility. There's a feeling of solidarity, so strong, very rarely achieved in many people's lives. Those are powerful incentives; after getting criticized by your boss, nagged at by your spouse, teased by your friends, finding a few hundred people to do something with, together, even for a few minutes, in a common (if inane) task, is invigorating.
The problem is this. A flash mob is still, fundamentally, a mob. Any research into the history and nature of mobs predicts that, sooner or later, things will get ugly. The cost of the damage caused by mobs, collectively, exceeds the benefit of the 'harmless (i.e. meaningless) fun' enjoyed by mobsters. In the long run, this will hold as true for organised mobs as for all other mobs. Proponents of flash mobs say "well, they're not mobs in the bad sense". But, a mob is a mob. You can't cut-and-paste psychological effects on a whim. The essence of what a mob is, is essential to the flash mob also. A mob is a mob, and supporters of current flash mobs are responsible for increasing the number and size of these mobs, bringing the date when things do get ugly ever closer, with less chance of putting on the brakes in time.
Unfortunately, it's all to easy for individuals to rationalize away their own personal responsibility. First is, "Well, I was only one person, I didn't really make a difference". Second is "I didn't know that I was giving moral support, and the illusion of popular endorsement, to a looter (or bully, or criminal) - I didn't even know there was one there!". Third is "I couldn't have known there was a looter there!". None of these hold up to scrutiny. Participation in a mob is the choice of each person, and responsibility for the effects each choice belongs with the person who makes it.
A peripheral danger to flash mobs is pickpocketing. Many many people gathered in one place, all of them distracted; there is no better place for a petty thief to be. A further corollary is that police presence is more urgently needed at the site of a flash mob, taking them away from more important duties they could be attending to. A known time and place of a thinly-spread police force is good news for criminals, bad news for everybody else.