[Disillusionist]

I can't always presume that, but for the sake of the thread, I will. I've seen decade-long games with great lore that still haven't bothered to proofread/typo their docs.

Before I get into the game, I feel it's required reading to at least go over the race I intend to play, quite possibly the race that is most likely to be antagonistic toward the one I'm choosing, the history of the area I'll be spawning in, and any unique class-based or world-based phenomena/conditions that would be common knowledge or fundamental to character development.

It's almost never a quantity-based response that makes me stop. If it's interesting, I'll read it all. I'll admit, I have about a five-page buffer for most topics before I say, 'Okay, I have a basic understanding, I'll read the rest later.' I can say that if it's corny, badly wordsmithed, or just stretches credulity, I will generally try to give it a chance to redeem itself and read the whole thing, but if at the end of a doc I'm rolling my eyes at the content or writing craft, I'm probably already on my way out the door. Website docs are the tip of the iceberg, and if they're not well-written, well thought out, or contain info that tells me that the game has elements that don't excite me, I might play, but I enter with very low expectations.
If I like the game, I'll have read all the docs in the first month.

Before. I hate looking like a n00b for missing a basic tenet of a world. Still, my strong opinion is that lore should be structured for expository both before and after first log-in, and that sparse up-front lore should be constantly, or at least regularly, added to.
Of course, no one should be spawning on their first day an utter expert on whatever part of the lore suits them best. There should be room for discovery post-login. I greatly prefer a very large ratio of post-login discovery to up-front common knowledge, because I'm going to spend much more time playing that I am website-cruising.
A bit atopical, but ...
If I find once I get in the game that the admins ignore or otherwise undermine their own official lore, unless there's a really compelling reason, I get pretty annoyed, like I would with any bait-and-switch. It doesn't take much effort to update a website if IC changes make lore docs obsolete or misleading. It's also well worth the time to make sure staff are versed in the lore base when training them. Mistakes or inconsistencies only compound if the official message is mixed or contradictory, unless that suits an intentional IC purpose.

This is a tough one. Both are important. Quite often, the mechanics are the tools whereby your character can survive long enough to begin interacting in a believable fashion, and reasonably often, some mechanics are directly derived from some lore-based necessity. Example: The world is PK-oriented. I don't want to bumble with my inventory because of command mistakes, and get whacked. Or there is capital punishment for theft. I don't want to type 'sneak' for my escape if the command set is 'hide' then 'skulk'.

Docs. If they exist, many times, they run counter-intuitive to modern impulses. Intuition can of course be refined, and I've relied on it at times, but it will only carry you so far. Oftentimes, intuition is baggage from experience with a differing set of game features from previous games. Now, if the mechanics can be easily executed intuitively, I'm a huge fan.

This is a list.

1) Poor word-smithing. It's a literary medium. If the docs are substandard literarily, I expect the same people who wrote them will be the ones in game providing the entertainment, and will be likewise substandard.

2) Shallow or corny docs. This is pretty subjective. If I'm rolling my eyes at some of the conclusions drawn in the expository, BIG STRIKE ONE.

3) Excessive attention to tangential or descriptive details. Let's say I'm reading key historical literature about an important figure. If by paragraph three, I know more about his wardrobe than his deeds (Again, sans a compelling intentional reason for that), I'm already skimming toward the end.

4) Published works-based games, VERY often, for two reasons. I don't enjoy games that push me towards literature written by someone with (usually) much much better writing skills than the game designers (especially if the game source is the first time I've heard of the literature), and hearing, "That's our lore base." For the same reason, I stay away from such theme-based games -especially- if I loved the source literature, since the gaming experience is -never- as good. I find such derived games to be crutch-reliant.
Peter S. Beagle's introduction to one of the LotR edition's forwards says it best, roughly paraphrased:
"We'd all like a shot at living in Middle Earth." And in my opinion, thus far, not one of us ever has, because the LotR-based games always fall far short of their lofty aim. There is only one JRRT, and he's not doing game design.
[Edited to add: I am -especially- scornful of games based on pre-published works that do not first obtain the author's permission.]

Conversely, I've played games where the completely -original- docs were as voluminous as many fantasy novels, and something in me responds to that level of creation dedication.

5) If I encounter none of the above, NOTHING will make me stop reading. I love reading great and unique lore.