The game really impressed me during the beta weekends (Rowan has a great preview up that summarizes my experience pretty well). I like the story driven gameplay, the atmosphere and setting, and the classless character development system. I also think that, much like LoTRO back in the day, a lot of the players that try TSW are not going to "get it." The game requires real patience to get into. For example, the quests. Each quest that you take opens up a chain of quests that together form a narrative. The best way to do quests is to take one quest at a time and see it all the way through to the end. Most of the time when you finish one quest chain, if you explore the area where you end up you will find an object or NPC that will open up another quest chain.
The game is not catered towards completionists. It's not all that likely that you will even find all the quests the first time you go through a zone, much less do all of them. I have read estimates that you only need to do 25% of the quests in a given zone to be strong enough to head to the next one (my guess would be more like 50%, but certainly not all of them). The idea is that each character through a zone will experience a slightly to extremely different set of stories.
Further, if you pick up a style of quest that you already have (e.g., an investigation quest), it will replace the previous quest of the same style in your quest log and wipe out any progress you've made on it. The one you ditched will then go onto a 24 hour timer before you can pick it up again. If you try to play TSW like WoW where you pick up every quest in sight and run randomly around the map to quest objectives, you absolutely will be stymied. On top of that, the puzzle quests can be quite challenging (and are meant to be). TSW is not a good game for those with MMO ADD.
The combat system and the character development system likewise need a real time investment before they start to shine. The emphasis of the game on narratives is also likely to turn off a lot of players, now that SWTOR has "proved" that compelling scripted narratives have no place in MMOs. This is an absurd contention in my mind, but a lot of commentators do seem to feel that way and have somehow managed to conclude that SWTOR settling at a mere 600-800K or so steady subs (my best guess, 1.3 million subs was the last number released) somehow supports this idea.
I don't expect TSW to set the MMO world on fire. However, I do think it will have very strong appeal for players that enjoy exploration, well developed narratives, and are looking for something outside of the fantasy / sci-fi genres.