From what I understand, if World of Warcraft has a direct competitor, it's Guild Wars 2. That is saying something too as just about every game that tried to face Blizzard's behemouth head on has failed quite significantly. Guild Wars 2 has hung in there though, building a dedicated and loyal consumer base over time.
Today the game's first expansion pack was announced by ArenaNet at a PAX South panel. Heart of Throns will take place in the The Heart of Maguuma, taking players threw three biomes, moving from the roots of it's trees to the very top.
To see some of the improvements made, take a look at this new trailer.
One of the interesting choices that ArenaNet have made is that there won't be a level cap increase, which is very rare for an MMO expansion. This means that gear and weapons that players have grinded for previously won't become obsolete when the expansion does come out (no date announced yet). That is a key decision and one I'm surprised more MMOs don't do. It shows a real respect for the time player's have already put in.