Tops:
1. Depth of the game: No matter the role you pick for your character, there is tons of stuff to do and ways to advance. Military, political, economical, religious, ...
2. Continuity: It's one long game with no end, nations rise and fall, nobles come and go.
3. Variety of realms and islands: If one particular corner of the game doesn't interest you, there's lots of other places to try your luck.
Flops:
1. Lack of player creativity: A lot of realms have no identity, they're pretty much just a bunch of players grouping together for the sake of having a realm. Alternatively, they are just carbon copies of existing game realms, or historical realms.
2. OOC unfairness: A LOT of realms are OOC cliques that are hard to break into unless you conform to the OOC-ness. Unless you're 'part of the group, your chances of advancement decrease dramatically. Other realms are just blatantly led by a bunch of RL friends who take over every position with a handful of people.
3. More OOC unfairness: A lot of the politics in the game are decided by a handful of players outside of the game (on IRC for example). Players who don't go to IRC often have little or nothing to say in their realm councils.