It's just tremendously unfunny--Cathy for nerds. Ignore the pandering, the in-jokes, and the unnecessary graphs, and what you're usually left with is poorly executed comedy. Look at this strip--what exactly is the joke here? "Sometimes people will attempt smack talk of the form 'Welcome to [bad]-ville/ton/burg, population: you.' Let us stipulate that this is a real place. That would afford me yet another opportunity to be a pedantic dick about something."
Well, that's not funny. It's nerd superiority syndrome expressing itself in a mundane counterfactual. It's just boring.
And then there's the art. What exactly the fuck is happening in that third panel? I looks like he gave up trying to convey what was happening there and just angrily scribbled something. Occasionally he attempts something more ambitious than stick figures, and the results are uniformly terrible.
Every now and then, a funny comic slips through--more often five years ago than today. But it's just brutal most of the time, as funny as getting punched in the stomach. He should have quit, or moved to a sporadic format, years ago. It's clear that most days, he has no idea what to do, so he just looks something up on wikipedia and draws something unfunny about that.
For more information, visit xkcdsucks.blogspot.com. Not nearly as good as xkcdexplained used to be, but if you read it every time you think you've read something funny on xkcd, you should be able to condition yourself into hating it as much as I do.
"From what I've observed some birds are pretty gay." ~ Evilyn