Mindlessly repeating internet memes for one (except in Soviet Russia, where meme repeats you). That's at least 50% of internet humor. Obscure references, as well. It's a type of humor that makes no sense unless you're immersed in the culture.
And even when it makes sense it can be pretty damned tedious. (Of course, for the "joke" to be anything other than self-indulgent the reference can't be all that obscure, so what's the point?)
The convergence of "geek culture" and "internet culture" is one of my big complaints. lolcats were funny for about 5 minutes.