People who stand around the machines and talk. Nowadays I just walk over and ask if I can work a set in. People, at least in the south, will become apologetic and let me get to work. One time I noticed a guy who would wait for me to finish deadlifts before he would do his pulldowns. Why? Because I was between the pulldown station and the mirror. When he did the pulldowns he would stare at himself. Now that I think about it the leg pad was against his back. He actually waited for me to get out of the way then sat down backwards on a pulldown station to watch himself lift. Amazing.
The curling guy could have just been looking for some space. I don't think he'll be back often. Most people just show up to gyms when they feel like it.
When I worked out at home I always disliked it when people wanted to work out with me. It slows me down and drags out the workout. I never was into partner training. I had to get used to working out in gyms. All of the activity is distracting and you're always dealing with people asking if you're using that, if they can get a spot, or making sure you don't back into somebody else. Not to mention having to track down the plates you need. A guy I know got a membership at my gym for himself and his wife. They both need to exercise if they don't want to die from morbid obesity and I'm enthusiastic for them but really, I don't wanna run into someone I know. Nowadays I move through the gym like I'm the only one there. The people are just fixtures to work around.
When I started using gyms I had to motivate myself a bit extra to workout. Its very convenient to have the gym right in your home. There's no need to drive there and back or find a way to integrate your workout with errands. On the flip side it can be tempting to put the weight down and go back to your couch, computer, etc. when your motivation is waning. Once you arrive at the gym there's nothing to do but lift.
This is kind of a standing joke on one of the other forums, and I always took it to be an exaggeration. But today I witnessed it with my own eyes. However, I have to admit that he had better biceps than I. But that's all he had - biceps. No triceps, no chest. Just hope he doesn't start coming regularly at the same time I'm working out. 
