The only problems are...
A) No RP at all whatsoever. Its Just an MMOG
b) you cant see people outside of towns or outposts, unless they're in your group.
c) hard to explore alone because of overpopulation of monster NPCs
But on the bright side, there are alot of expansions.
Any of the three can run as a standalone game. If you want Content adn story line, go with Guild Wars: Prophacies
If you want Faction battles, war, or anything oriental, go with Guild Wars: Factions
If you want a decent storyline, adventure and trainable minions (heroes) then go with Nightfall.