I JUST finished factions for the first time with one of my characters two days ago, and I still want to finish it with more characters (not to mention I'm still trying to finish prophecies. Fire islands is hard, enspecially to get a group that doesn't all leave half way through the mission.) I don't think this is too soon though. It means more to explore and fight through ALONG with trying to beat the other two chapters. I personally like them going a little too fast with these, because it means I won't get bored of it, unless I get bored of all of GW, which doesn't seem to be happening anytime soon. Now, if I get stuck on a mission, or bored of a certain chapter, I can always go to another one and play that untill I fell like trying the chapter I got stuck on/bored of.
The only problem with this coming out with expansions is that there are already some missions in cantha and tyria (this problem is mostly in tyria) that are hard to get groups for because there are so few people there. With more chapters, that means more towns and areas for people to be in, which means people will be even more spread out. That will make finding groups in some areas even harder and most of the missions towards the end of the game are literally IMPOSIBLE with henchies, so you HAVE to find a player group.
Plz arena net, MAKE HENCHIES SMARTER/BETTER!!! I wouldn't be stuck in fire islands if I could do the mission with henchies because they don't randomly decide to leave in the middle of the misison, plus, they are ALWAYS at that place and there are sometimes when I go do a mission, but I can't because there aren't enough players to even form a full group (most of these players are afk too) and the henchies are too pathetic to make it half way through the mission without having 60% dp so I have to go do something else (it has happened to me a few times before, but it is very rare. You would think more players would be doing missions in the middle of the day

)