Things you can receive warnings for:
NSFW content in posts that's untagged
NSFW Avatars
Spamming the hell out of the forums (seriously, 21 replies to yourself in order to up your postcount?)
Personal Attacks
Warnings degrade over time, depending on when your last warning was. The warning punishments are as follows
no punishment
no punishment
no punishment
1 day readonly
2 day readonly
7 day readonly
14 day readonly
30 day readonly
90 day readonly
There is also discussion about implementing a mod queue after your third warning, but this is not implemented. These punishments may be revised at a later date depending on admin feedback.