They can be used for a variety of exercises: bench presses, squats, deadlifts, overhead shoulder presses, cleans, you name it... But one thing they should never EVER be used for: barbell curls of any kind. First off, no one's--NO ONE'S--arms are strong enough to actually need a full-on Olympic bar for curls. It doesn't matter if you're using the bar on the bench, or the bar in the rack--it's uneccessary and rude. It doesn't matter if you're doing wrist curls or biceps curls--there are fixed barbells for that! And if you're one of those people who likes to stack two or three ten-pound plates on the ends and do dropsets, there are mini Olympic bars for that too. The rack needs to be respected and, along with the Olympic bars, reserved for people performing exercises which actually require that equipment.
2. Super-Setting: There's a Right Way and a Wrong Way
Super-setting can be highly effective as a plateau-buster and overloading technique. But you MUST be aware of the scarcity of resources when deciding on the two exercises you will be performing. If the gym is empty, go ahead, do a super-set in which you bogart the rack and the smith machine--but otherwise? Bad form. Especially when it comes to leg equipment, try to be aware of the fact that there are not many options when all machines/racks/smiths are in use. Furthermore, use of machines for exercises which can be performed elsewhere should be done with extreme care and courtesy. Think to yourself before claiming a piece of equipment: Do I really need more than 110 pounds for this exercise or can I just as easily use a fixed bar? Can I do this exercise somewhere else or do I really need this machine? In most cases, super-setting on scarce resources is a total d-bag move: the squat rack should rarely, if ever, be used in a super-set--if you do insist on using it for one, be extra accomodating when someone wants to work in--ideally defer to the other's use of the machine whenever possible, especially when unique, unduplicable machines and equipment are involved.
3. Stretching: There's a Time and Place
Between set stretching is not only a great idea, but an imperative. But what about the I'm-tired-and-need-to-lay-down-at-the-end-of-my-workout stretching? This type of stretching should always be done outside the weight room. In any case where you need to sit or lay down, grab a mat and beat it! People lifting should not have to worry about stepping over or around someone who insists on stretching their hamstrings right next to the bench press. The floors in weight rooms are usually pretty nasty; you're better off in the cardio or group fitness area anyways!
4. Access for All
In line with the previous point, everyone should be able to access everything (which the obvious exception of machines already in use) at all times. This means no repping out RIGHT in front of the dumbbell or barbell racks. No stretching RIGHT in front of the racks, no sit-ups RIGHT in front of the racks. Push your benches far enough back so that people can use the dumbbells in front of you. Be considerate of those around you: is that guy trying to get past you to grab a couple dumbbells? Yeah, it's annoying to have to move right in the middle of your set, but you're the one who posted up in the way.
"Working in" can be a frustration for both parties: you have to remember to reset all your seat, weight, and other settings and you might have to go slower between sets than you would like--any way you look at it, it's cramping your style. You might get really unlucky and allow someone to work in who then forgets they were working in, starts talking on the phone, and takes up WAY more of your time than was appropriate. Well, you just have to remember not to let that person work in next time. Most of the time, however, sharing is absolutely necessary. Unless you train at Gold's Venice (which has multiple large warehouses full of awesome equipment), there's usually only one of the machine you (and someone else) are trying to use. When working in, it's important to remember to ask first whenever possible, limit lolligagging to the absolute minimum, and try not to unduly restrict the other's workout--hanging around watching them in order to communicate impatience is also generally not appreciated. Let the other individual focus during their sets the same way you will want to during yours. If we can all share, we can all get a great workout!