Gravity is not allowed for the reasons Joker stated. The server often needs to restart after someone fucks with gravity because it's buggy. If you couple low-grav with parachutes it gets even worse.

Slap is another command that shouldn't be used (frequently). It should only be used to move someone out of the way or as a warning and should never be used to, say, knock someone into an area that would get them killed. 'slap @[Ts/CTs]' and 'slap @all' are completely restricted.

Freeze is pretty much restricted because as a human it prevents you from moving, often resulting in getting tagged, and as a zombie, it prevents you from tagging. The only place I ever use freeze is on Moria to protect Gandalf (and that's only after the bridge has begun to collapse).

Rename is not currently allowed because name changes are permanent (until map change). I wouldn't use this on someone unless you're sure they're okay with it. Even then, use caution.

In essence, use common sense. If it looks like abuse, it probably is.