A MMO basically anyone can get into!
The good: World of Warcraft is a very solo friendly game. Every character class can level up to 60 by themselves, though they should group together for tougher quests such as those located in dungeons. In groups, there are no "useless" classes. Each class has a purpose, such as healing, doing damage, taking damage, and supporting the group with various abilities.
The PvP (player vs. player) system now has a purpose beyond just slaughtering each other. Every time you kill a player of the opposing faction that is within your level range you get honor points. These honor points are tallied up every week and calculated into rank. Each of the 14 ranks, with military names, has various rewards you can purchase in the form of equipment that will further help you in PvP combat.
The best way to earn honor points is to enter one of the three battlegrounds. The first battleground, Warsong Gulch, pitches 10 Horde players against 10 Alliance players in a game of capture the flag. The second, Arathi Basin, is all about controlling resource points. The team that gets 3000 points first wins. The third, Alterac Valley is an all out war against the opposing team with 40 players on each side with the goal of destroying the enemy's base and killing their commander.
The bad: PvP servers have the problem of ganking, where high level players will go out of their way to kill low level players who stand no chance against them. This is probably why some people perfer Player vs. Environment (PVE) servers where players go into PVP combat when they want to.
A piece of advice to those starting on a PVP server: avoid Hillsbrad Foothills! It is a source of constant skirmishes and ganking between both factions.
Another problem is that when you get to level 60, most of your dungeon options are 40-player raid instances which take a long time to go through. If this timesink doesn't appeal to you, then you should focus on the few top level dungeons that have a group limit of 5 to 10 players. Either that or focus on gaining rank status in PvP combat via the battlegrounds.
The only other problem isn't the game, but the servers they are hosted on. Many servers are queued up in the evenings and weekends, forcing you to wait in line. Not only that, a few of the servers have massive amounts of lag from time to time. Fortunately, Blizzard is going to upgrade all the servers in April this year.