Rules were imposed but they were not defined in a list. If you look around, you'll find a lot of instances where theymos had created a thread in which he posts a new regulation that is to be followed: eg. altcoin giveaways, doxxing hence why the mprep thread is named the "Unofficial list of
(official) Bitcointalk.org rules [...]"
Rules were set in place before 2013, obviously.
The policy to not remove anything worked when the forum was small. Now that we have thousands of posts a day, we can't afford 50% of them being junk. The moderators are now instructed to be less tolerant of low-value posts.
Some guidelines:
1. Free speech - you can say anything as long as it is relevant and presented in a calm and polite manner. Swearing, SHOUTING etc. make your post more likely to be removed.
2. No zero value posts or threads, like "SELL SELL SELL"
3. No pointless or uninteresting threads.
4. No referral code spam
5. No NSFW content
All you have to do is look around. mprep details sources in his 1st post.
