My personal experiences and preferences
My first pokerroomI played my first play money games on a site called HoldemPoker.com because a colleague of mine recommended it back in 2004. It was free and without download requirements and at the time I didn't have to much confidence neither in the poker industry nor in my own abilities as a poker player. Later they merged with PokerRoom.com and I automatically became a member there. That’s probably why I started looking at them as a possible place to make my first deposit and in the end the deciding factor was the FDB and the fact that they accept instant payment with credit cards and also transfer the money back to your credit card on request. The remaining questions wasn’t that important to me at the time, but seen in retrospect I wouldn't choose PokerRoom.com again.
My current and preferred pokerroomsI have quit playing on PokerRoom.com since I don't like the tournaments they are offering.
Another pokerroom I frequented a while was FullTilt.com (recommended by the same colleague who recommended Holdempoker.com), but my main site at the moment are by far VCPoker.com and secondary TitanPoker.com and they offer some of the best low stake action I have seen so far. They also offer very nice FDBs, where VCPoker will give you $10 instant cash without any deposit, no strings attached, ready to use in tournaments and ring games plus $25 in bonus that you need to "play free". Titanpoker.com requires a $20 deposit, but then they will give you up to $50 instant cash and $50 bonus.
Both offerings are very nice - for beginners I think VCPoker is the absolute best choice, since you can try out the action without depositing any money at all.