1. Paladin has a very good ratio of difficult (easy) vs. raid usefulness (brings a tons of buffs, and being strong altogether). People look down on it as "the easy/noob class", but it does offer a lot of depth and people always love to have at least one of them in their raid (regardless of spec).
Marksman Hunter is also fairly easy to play and is pretty popular for raid comps.
There are a fair number of "really easy" specs to play, but they aren't nearly as popular for raid compositions.
2. Honestly, I would encourage you to eventually play a ranged DPS, melee DPS, a tank and a healer. Eventually, of course. Not all at once. Doing one of each and learning how to play them and how they work will teach you a lot about how a variety of players play the game and how some things work, allowing you to make the lives of other players around you easier.
As for which to choose based on mechanics, many of the specs have a variety of mechanics that are unique to spec or class. So it's hard to say, really. I'd have to stay refer to #1 for a starting point.
3. One of the most well-rounded and strong classes in the game. PvE-wise, anyways. All 3 specs are something your average raid leader would love to have. Not necessarily all at the same time for smaller raids, but still.
4. I'd have to say Arms Warrior or Mage in PvP. Largely because of how they play and how difficult they can be to master. Also, if you plan on PvPing, I'd strong recommend having at least one healer character that you enjoy to PvP on - there are never enough healers for battlegrounds.
5. There isn't a "PvE realm". Both have strong PvE communities, that is for sure.