Nothing.
Not really, but sorta.
Consider everything you know as a percentage of all that there is to be known. In this sense I know 0% of everything. I will never know more than 0% of everything that can possibly be known. I'll never even know 1% of pi. Maybe I'm looking too far into nothing, but I still find it a bit alarming.
Many people spend their lives, or at least a good portion of their lives seeking knowledge of one sort or another. I'm not going to say it's in vain, because knowledge is powerful and important, but does anyone ever succeed in this quest? No matter what you learn, you always yearn for more. No one is or ever should be satisfied with what they know.
Now think about a supposed all-knowing god. All-knowing. What does that mean? Such a god must have infinite knowledge, literally. There is no bound on what can be known. Going back to pi - it never ends - proof enough that an omniscient being would have a literal infinite amount of knowledge. What's the 1,000,000,000th digit of pi? He (or she) would respond immediately, without the need to think about it, he would just know. Or would he answer before I even asked aloud? He knows everything after all, even what I think.
Does he know the next sentence I'm about to type, even when I don't? This brings me to another point. Would an omniscient god know everything that is known now, or everything that is known ever? What I mean is, does this god know things before they happen? If I'm going to fall down the stairs on my way out of work, does he know this in advance? If not, then by acquiring such knowledge he is adding to the knowledge that he possesses (I know, infinity + 1 is still infinity), and if he knows more now than he did before, was he really omniscient before?
If this god is all-knowing, and he created me, then why aren't I perfect, why do I make mistakes? If I set out to make something, I would certainly make it perfect if I had the ability to do so. People say "in a perfect world..." Well if this god created this world, why isn't it perfect?
I tend to go too far with things like this, and it's getting moot, so I'll stop here. No moral here, but plenty to be learned ;-)

