Be polite and don't barge into people's private rooms uninvited.
Spying in-character is fine, and a part of the genre. If you want to spy on someone IC, you should talk to the player you're spying on OOCly, and negotiate the terms. The other player may choose to send you roleplay logs, allow you to place a Puppet-flagged object in his scenes, or something similar.
Other types of eavesdropping are a violation of privacy. This includes Dark-flagged listening objects, command-trappers, and a variety of other MUSHcode-based tricks. If you are watching someone — whether they're roleplaying or chatting out of character — you are OBLIGED to let them know. (The staff does not eavesdrop on players, period.)
Some areas may be built in such a way that conversations are forwarded elsewhere — the use of Audible-set exits to allow, for example, a "stage" to be seen by the "audience", is quite commonplace. Use the '@sweep' or '+awake' commands to check what in your location might potentially be listening to you.
The identity of other people is their own business. If someone tells you any personal real-life (RL) details, it doesn't entitle you to tell these details to others. Altchars are often sensitive information; if a player doesn't want his known, you are expected to respect that desire. Similarly, you should never press people for details of their real selves. If they want you to know, they'll tell you. At the same time, never assume that people are who they say they are. They may not be of the gender that you think they are, they may not live where you think they live, and they may, indeed, be nothing like what they say they're like. The staff will never give away this kind of information; don't even try to ask about it.
Descriptions, code, and other things that are part of the database are considered private. Don't copy them without the explicit permission of the original author.
Don't try MUSHcode tricks to search the database for information about other people's stuff, or attempt to circumvent the security of someone else's objects — this is against the rules, and will not be looked upon kindly by the staff. If you have reason to believe that there are problems with someone else's stuff, notify them or the staff.