Don't save treadmills - It is really unpopular and quite selfish to claim a treadmill when you're not even using it. This may involve putting a towel over it so no one else can use it. This is similar to what people do on vacation when they put their towels on the sun beds in the morning. Even if the gym isn't too busy this is not a good idea to do.
Leave some space - This is an unwritten rule and is like invading someone's personal space. If the treadmills are very close together and there are plenty free then don't go onto a machine that is directly next to someone using one. If there are 8 treadmills and only 1 person using one then leave a machine free next to you rather than going next to someone.
Don't stare - It can be very tempting to look at the person next to you and their console to see how fast or how long they've been running. But you really shouldn't do it, you may want to strike up a conversation and have a peek when you've done this but staring at others while they're exercising should be avoided.
Exercise properly - In the gym I go to there is always someone who scrapes their feet along the belt when they walk or run. This creates a screeching noise that is really annoying to others. It's bad enough hearing some people grunting but a loud high pitched sound coming from the treadmill belt is very off putting so don't do it.
A lot of these you will do without realizing as we've all got to take care of others when using a gym. Even new starters will pick up these rules whether they're written or not and a little reminder if someone is hogging the treadmill will often work wonders. This helps to keep the exercise flow of people in good order.
If you're a member of a gym with lots of treadmills then you're in luck. Many of these rules are not needed. Time limits may not be needed as well as saving the machines. Wiping down is something you will have to do and you might need to bring a small towel in especially for that purpose. The best thing you can do is enjoy your exercising and be considerate to others.