People in general like to make up their own rules. Keep in mind that even with all the people on these forums, it still only represents a small cross-section of the JK2 gaming population (the "community").
The only real "rule" that's hard and fast is that the person running the server (the "host" or "admin") makes the rules. Respect that.
If you want to learn how to play the game in MP, first read over the manual and readme.
Then I suggest heading over to http://strategy.jediknightii.net
(probably a good idea to bookmark this site) ; )
If you join a strange server and wonder what the rules are, just ask the host/admin, if they are a worthwhile host, they should explain them to you if you ask politely. Personally I prefer servers without a lot of rules, but that's just me. Sometimes it can be more fun though... depends on my mood. A lot of people complain on the forums about this or that, but I know in practice a lot of people DO just "play the game" and have fun. Look for those people and add 'em to your favorites list, my advice. = )
Whining isn't really right or wrong, I just see it as a total waste of time, both to do it and to listen to it. I feel that People ought to just get that stuff out of the way before the game, but I can see it having a place if somebody was actually cheating or something, but it's the host who has the reponsibility to do something (or nothing) about it. I figure a decent host would take more kindly to constructive criticism than flames and people cussing at him/her. After all, nobody forced them to join his particular game!
People can always be polite and find another server that's less laggy or more to their liking with the settings or "house rules."