I actually like it but the game is definitely not for everyone. Unlike WoW, most of it cannot be done solo and I think it gives the average intelligence of gamers too much credit.
The instancing is kind of annoying but I think it beats the alternative of massive KS'ing and looting. Also, there are story based missions that wouldn't work if people entered and exited them at random.
The game is also more tactical in battle than WoW which is probably why it isn't popular amongst that crowd. Battles have a lot more to do with which skills you have equipped (you can only have 8 accessible at a time and switch outside of combat zones) and how you use them then level. You also can't get very far on your own. In the original, a warrior with monk as their second class could get through 3/4 of the game solo, but Factions has rebalanced it quite a bit so that is not longer possible.
Well, it gets larger as you get farther in the plot.
Starts at 2 in the original game (though that part is short, starts at 4 in factions), then 8 and 12. I think you only get higher than 12 in PVP and Guild battles (except for a few Factions missions that have 16. Factions also has team competitive missions with 12 players per team)