I think, aside from MUD rules in general, the real impact it has on a MUD is by who is administrating. In MUDs I've played, you can usually tell from the first deity contact you have whether or not they are professional... I use professional in a broad sense. Some games aren't run that way at all, but just a free for all funfest for the most immature kids to come on and mudsex all day.

One MUD I played specifically, while the actual game itself and players were really fun, the administration and pantheon was absolutely horrendous. They basically drove away their entire playerbase, and I was one those players. It had nothing to do with the rules in particular, because they DID have a HELP file for rules. But, it seemed like everything we players did, the moment someone complained, you were getting reprimanded for some ridiculous reason.

i don't entirely agree with RULES being spammed whenever someone logs in. In my experience, I find having to read through pages and pages of help files before I actually get INTO the game itself to be extremely boring. My current mud doesn't spam you with rules. It just takes you through a few novice guides and then you join your city and guild. Any rules you have to follow are defined by the city or guild you join, because they have GHELP and CHELP files. Most have a rules section. Some are a bit more free. All are attempting to encourage roleplay.