It seems that I am not the only one thinking about the issue of XP and levels. I am not really comfortable with the way Lamentations of the Flame Princess is handling XP. You get them for treasure, but relatively few for slain monsters. Partly because in Lotfp you're not expect to slay that many monsters.
Handing out XP for treasure has its own issues. Because suddenly I feel compelled to place enough treasure everywhere so the PCs can level up. To me that's not helpful. If they don't find the treasure, they don't level up and I can't bring on greater challenges. For my group that's further complicated by a player pool-style of the campaign.
Therefore, I need another solution. Either I am going to hand out a large sum of flat XP each session. So that you can be sure if you show up, you'll get maybe 750-1.000 XP per evening plus XP for monsters. Also, I tend to award additional XP, if you come up with something game-related that makes the game more fun for the other players or the GM. That could be, but is not limited to: a character portrait (either self-drawn or found on the internet), a character backstory (nothing fancy, just a couple of sentences describing who you are, where you are from and what you want) or even a session log. That would fit right into an XP model. Say 1.000 XP for attending and 500 XP per thing you do for the game.
That's huge chunks of XP at lower levels. But that's me. I want to play through the low levels, but not get stuck there for ages.
The alternative system would be to award a new level as soon as the character has participated in a number of game sessions either equal to his level, equal to his level +1 or even double his level. Thus honoring the investment of time and getting each character to advance roughly at the same time. I am not worried about game balance for the same reasons the blog post mentioned earlier isn't. You are hyper-specialized. You can as well get better at the same time everybody else is.
But currently, I think I am going with the flat XP model for honoring the time spend in the game.