I'm a big fan of games like Entropia Univeralis and Hearts of Iron 3, as well as their developer, Paradox Interactive. Sure we've had our ins and outs, but their games are very solid, fun, and deep. There's one huge problem with them, in my opinion, and that is multiplayer. The problems with multiplayer stem from two areas: length of gameplay and no defined goals.
Length of gameplay isn't always a bad thing, but given that Hearts of Iron 3 runs from 1936 through 1948, there's a lot of game there. Most multiplayer games start in medias res between 1939 and 1942, but that's still a lot of gameplay. Add in the need for a constant speed to facilitate multiplayer, and you have a lot of time commitment for multiplayer to work. The large scope of the single player game is the best part, in my opinion, and multiplayer just makes that drag on. Usually playing single player I will divide it between several long gameplay sessions, or dozens of shorter ones. Entropia Universalis is even worse, as its timeframe spans four hundred years.
Lack of defined goals is another big issue for multiplayer. It usually comes down to eliminating all the other players by any means necessary. Hearts of Iron 3 gets around this by assigning goals to each of the factions, which helps somewhat, but Entropia Universalis doesn't have that benefit. This means that multiplayer games drift between aimlessness and utter destruction. Which is a slight bit better than games where one side gets an advantage and the other side is doomed from there out.
That said, the time commitment is a worse problem than the lack of goals. Players can always come up with goals, but they cannot do much to limit the time needed for the game. Sins of a Solar Empire suffers the same problem, even though it is a fun game, multiplayer is torture.
Just some thoughts