Ok, so I'm gonna assume that everyone knows it's not ok to do stuff like fake tap, wear a smelly gi, purposely hurt someone etc... and I'll just concentrate on stuff I know has come up regularly in questions people have asked me over the years. The most important rule to remember is that you should try not to injure anyone in training, so it's always better to miss a submission than to crank something on. This really does fall into the obvious category though, I think.
So, what things do people think might be not ok in training? I've heard people consider stuff like shoulder pressure/head control, knee-on-belly, hip pressure from mount and similar all things which shouldn't be used much in training. Straight up, I find this CRAZY. All those things are good jiu-jitsu, they are the proper way to control people, you should definitely use them and learn to get good at using them. Just think about why you use those techniques; to allow you to advance position or secure a submission. If you just want to pin someone for a round (to get better at control, which is a fair goal at times) then pick someone tough... a 90kg purple belt pinning a 70kg white belt for a round with brutal shoulder pressure is uncalled for, but if they can do it to another purple belt it's fair enough. Similarly, if you have position on a higher belt then any amount of pressure is fine, if they don't like it they shouldn't have let you get the position or should be able to escape. The only thing I've ever told someone applying shoulder pressure to me is that they should apply more pressure!
That covers off applying pressure/discomfort. I just think, if it's legal under IBJJF rules it's definitely ok in training. Consider the difference in ability and strength/size when you roll as normal and you should be fine.
What about other stuff? "Illegal" submissions is a big one... most people consider the IBJJF rules to be the standard for what submissions are legal at each belt level. Personally, in training, I don't mind people doing any submission at all as long as they control it and always give their partner time to tap. If you can't control it enough to give them time, don't do it. If your partner doesn't seem to want to tap, tell them they need to because they are caught, don't just slam it on. If it's a submission they might not be familiar with it's always better to let it go than hurt someone. For heelhooks I would suggest only ever training them with a catch and release method, they are too dangerous to go for the finish in training unless you're rolling with experienced training partners who will recognise and tap to them.
Then there are submissions which are applied slightly "wrong". Stuff like having a gi lapel wrapped across the mouth not throat/neck. As long as you give due consideration to the level of your partner, I have no problem in submissions like this. At the end of the day, if someone has to tap they have to tap. These things can happen in competition so everyone needs to get used to it in training.
Beyond that I don't think there is much else which falls into this category. The only other things I've come across are a very small number of people who try to gain an advantage in training... wearing a really tight gi, or resting every other round etc... this isn't stuff which is that bad for other people, but it does show very weak character and a poor mentality of seeing training as competition.
So really, it's easy. As always, you should consider the level of your partner and any size/strength difference, and remember that above all else you should try to avoid injuring anyone. If you think like that, you can't really go wrong.