Okay, here goes....
1. DAoC is a massive multiplayer online game. There are generally 1500 people on a server at a time. They are divided into three realms- Hibernia, Midguard and Albion. Therefore, you are playing with about 500 people at a time in one world. The three worlds or realms will come together to war or fight against each other at various times. This is the player verse player element.
2. There is some role play involved. It is not manditory, but welcome. YOu can develop a back story if you wish or you can just sorta be yourself with in reason. Depending on the server you choose will depend on the depth of roleplay. Our group plays on a Roleplay server so there are less Doodz and l33t people.
3. The gold and platinum editions have different add ons. I would buy the platinum if I were you. That will give you the ability to have all the expansions. However, if money is an issue Gold edition is fine. The final add on is for players above level 40 which can take a month or more to get to if you play a great deal.
4. Yes, there is a monthly fee for the game. I believe it is around $12. The first month is free. I think it is worth it. That saves me from buying a new game every few months. Plus, Mythic does a fairly good job of updating issues in the game. There is another expansion coming out for Christmas, so I imagine that the updates will slow until after the first of the year.
5. If you liked UO, EQ or AC this game is very much like them. However, it is better than all three with more depth, better enviorments and excellent player verse player activity. The PvP is considered DAoC finest aspect by the gaming world.
I have been playing for more than two years (in two different settings). I have been with KoT and Albion for 18 months.
I would give you a small warning. There are many new games coming out that will make DAoC a generation behind (that is why they are adding a new expansion). But, I do think the game still has a great deal of life left if you enjoy it. I still play for the relationships i have developed in DAoC and in the realm itself. So, if you like community this is a great game for you. If you like to point and shoot, i hear there is a new quake out...