People on the dance floor who don't dance

The problem with standing on the floor is that others will inevitably bump into the stationary person. When two people collide while dancing they generally see it as an inevitable consequence of moving around in a crowded space. One would not stand and talk on one's mobile phone in the middle of a crowded sidewalk, so why would one stand in the middle of a dance floor?

Here are some possible explanations for this phenomenon:

The person is lost

Plausible, but standing on the floor is probably no more effective for spotting wayward mates than standing at the edge.

The DJ is awful

This was not an issue tonight, but people have a legitimate reason to stop moving on the floor if they think the music is bad. This does not generally create a problem because the music is always adequate if the audience is moving and happy (c.f. middle school "dance").

The standee is tripped out (on drugs, alcohol, life, etc.)

We all know it happens, and this is a somewhat legitimate reason to be standing. In fact (at least in London) random people will start inquiring if you are all right should you stand motionless against the sea of sweaty bodies. In this case the standee usually just needs prompting to break their trance, and all is well.

The person is "tired"

At the end of the night some people do not have the energy to keep up with the terminal crescendo. Being "tired" sounds plausible to the uninitiated, but devoted club-goers know that this is no excuse for being an obstacle on the floor (the floor is probably the worst place to recover). Again, one would not stand in the middle of a crowded sidewalk just because one was tired. "Tired" people are often making an excuse for being jealous or possessive of their more energetic significant other, thinking that by shadowing him or her they will curry favour. Depending on their relationship this might be productive, but is embarrassingly selfish if there is not room on the floor (hence the rationalisation).

The person is awestruck by someone's moves

Beginning dancers will stop moving to admire someone's floor work, this is about as tactful and welcome as standing in a crowded sidewalk to stare at a parked Ferrari. Emulation is the best form of flattery, so watch and follow. Don't bite moves step for step, but show your respect (deference?) without interfering with others. Just because one has never seen break dancing before doesn't mean everyone else on the floor wants to stop and watch.

The person just realized they can't dance

One should always remember that most people don't care--I spent 30 minutes bumping against a guy standing in the middle of the floor who was staring at the DJ. He had been flailing a bit earlier, but must have decided it wasn't worth it. Even one's worst attempts at moving show more positive attitude than becoming a bumper.