Sorry, but i disagree with some points made here. (Agree with most).
1) I think the early days of UFC were alot closer to a street fight than now because of the rule changes, such as time limits and breaking up a guard position for crowd enjoyment.
2) akja has a good point about the fighters overwelming their opponent. That is probably as close to a street fight as you can get.
3) Tulisan said most people on the street are not as well trained, that is correct, so alot of people will just rush in as fast as possible, trying to do some damage, completely ignorant to what their "trained" opponent is about to do to them. As far as weapons, that is a factor, but not a huge one. Most "untrained" people really don't think to pick up a weapon simply cause they don't see "weapons" the same as a "trained" people, and if they did pick up one they would probably just injure themselves. lol. Also, unless you are trained (or maybe disturbed), it is not really human nature to use a weapon against another human being. (thus the slash wound instead of stab seen so often in knife attacks).
4) Going to the ground: alot of street fights do end up on the ground. some falls, trips over something, knocked down by opponent, etc. or like tmonis said, some one might go to the ground and curl into a ball. In most instances, if one goes to the ground, then the untrained opponent might see it as him winning and jump on you.
O.K. to get back to the topic (sorry) the UFC is fairly close to a real street fight in the way of the fighters rushing one or the other, and a lot of them ending up on the ground.
Respectfully,
Shane