If there is one thing I would have liked Legend of Grimrock to have, it is a proper New Game+ mode, where I could bring in my experienced party of adventurers and continue to level them up through the dungeon again, facing stronger enemies. I see the flaw here though; Grimrock is not Diablo, and instead of using randomly generated dungeons, it relies on a single, meticulously constructed and labyrinthine oubliette as its setting. The puzzles are half or more of the experience here, so for an additional adventure to really be great with your existing party, it would have to be through a new dungeon.
This is not to say that Grimrock has no replay value built right in (in addition to the user-made and official expansions that are sure to come), as there is a novel take on the idea of a second quest for the game; it's just that that particular mode does not import your existing party when you play it. If you would like to replay through the same dungeon, though, there are plenty of potential party configurations to do it with, between the four races and three classes to choose from for each party member, and even further decisions about which skills to level up, and what weapon types to use.
The key limiting factor here are the levels. Grimrock's main campaign has 10 full levels and 3 smaller ones toward the end of the game, which is just about a perfect amount for a single playthrough. I have to admit, though, that I was expecting to carry my party and all their gear into a second playthrough, which isn't happening until and unless expansion dungeons allow for it. I hope more about these comes to light soon.
More levels will help to better balance the class utility for the game, also. This thread on the Grimrock official forum points out a few concerns people have with the way things currently are. I used the default party lineup, so I didn't really encounter too many issues, incidentally.
So, the point of this post is just that I've finished Legend of Grimrock, and really enjoyed it. I think it's got a good future ahead of it, as well, as it appears to be highly modifiable, and games of this sort are nearly non-existent these days. If I had infinite time, I'd try a playthrough of the stock second quest mode, as that seems interesting. I don't though, alas, as I'm flying to Japan tomorrow for a couple of weeks.
Look for more impressions of Avernum and other ipad games soon!