Fun fact: Libris Mortis has a 100% canon adept lich. They even refer to her as 'the weakest lich in existence'. I'm still waiting for a magewright lich, though.
Liches, as I'm sure people know, are rejuvenating spellcasting undead. Becoming one grants small bonuses to all mental stats and a reasonable natural armor bonus. The fear aura all liches have is less interesting, as very little at ECL 11+ will have less than 5 HD.
A lich's touch attack is worth mentioning as well. It deals a small amount of negative energy damage, but also forces a save vs. permanent paralysis. Sadly, there aren't really any first-party ways to use it at range, and for most liches (ignoring the occasional duskblade) melee isn't a good place to be.
Furthermore, liches gain a number of passive bonuses. DR 15/bludgeoning and magic is reasonably nice, cold and electricity immunity are sure to come in handy a few times, and skill bonuses will probably see some use as well. The specific immunity to mind-affecting attacks is weird, though.
Finally, there's the rejuvenation ability. It's a bit unclear how exactly it works, but basically it returns the lich to unlife 1d10 days after its destruction. I don't think this affects the LA: usually a level 11+ party will have ways to return people from death faster, even if they are undead.
I'm considering both +2 and +3 LA here, and think I'll go with +2. Lichdom does grant a number of interesting abilities, but will probably result in a net loss in casting ability (which most people at CL 11 or more are attached to).
Note that becoming a lich is very pricy. Without a way to get that cost down somehow, it may be more like +0, or even -0. Remember, kids, gold is good!