Specifically, you are not allowed to give people any incentive to post insubstantial posts in your threads. You can't offer to pay people who post their addresses, usernames, etc. You can do giveaways off-site and link to the giveaway page in a thread, but you can't give people any bonus for replying to your thread.
Similar threads are already restricted to Games and Rounds in the non-altcoin sections, but the giveaway-related post volume is so high in the altcoin sections that I've decided to just ban them entirely here.
This is the statement for Bitcoin's Marketplace Section:
Rizzrack is right although I think that posting giveaway threads in the games and round section is still in a case-by-case basis. The mod reviewing this thread might have given them a free past as some of the process in their giveaway are done externally and is not mostly about incentivizing the post itself.