Either way, I'll split this up into 3 sections : The Criticism, the suggestions, and the praise.
The Criticism:
To start us off, we have another instance of the head not being attached to the body in panel 1.
The setup for the comic isn't so bad - we see a man rock climbing, plenty of opportunities to go from there -... And then in the very next panel, we see pieces of someone. Please - don't give away what's going on in the second panel when you're aiming for a big surprise. I'm biased though, having already seen the fourth panel - it's entirely possible that someone reading it for the first time could mistake the leg and face for that wooden post that is the goal in kids' rock climbing course.
Speaking of a kids' rock climbing course - that's what this is supposed to be a picture of. This stick-man is using the almighty power of slanted photography to make it look like he's climbing a kids' rock climbing course. Who among you would be truly impressed by that? Why would anyone even want to post pictures of themselves climbing a kids' rock climbing course?
Admittedly, the fake picture would be made a lot more impressive by the fact that he's not using a belay (the device that clips to a rope/pulley system and supports your weight), except for the fact that professional climbers, the kind who climb Mount Rushmore [people climb mount rushmore??? -Carl], would still use a belay on a kiddy rock climbing course because most of them aren't fans of falling. I know I'm nitpicking, as it's supposed to be a falsely impressive picture, and, you know, the character isn't actually climbing.
And finally, we have the alt text. This alt text wouldn't be so offensive if the comic it follows was actually good. But it isn't - it's mediocre. And in the same way, the alt text itself is mediocre. It's a problem I feel we see a lot in xkcd - the alt text continues the already boring conversation. If the alt text is a followup to an interesting/funny conversation, that's great - but in this case, it would have been far better as a random observation - or perhaps a callback to some of the old alt-texts, with Mr. Munroe (Randall?) making a comment in the first-person about similar situations/what he would do in the situation.
Either way, it all wraps up with a mediocre punchline that doesn't add much to the comic - even omitting all the words and keeping just the "art" that's currently there would make for a better comic.
I'm worried that I'm criticizing just for the sake of criticizing - but I hope I've raised at least a few valid points.
The Suggestions:
Put the head on the body in the first panel.
Erase the girl in the second panel - although, admittedly, if she just appeared in the third panel, that would be a little weird. I stand a bit divided here, I guess.
Give some funny alt-text.
The Praise:
Well, I guess more effort was put into the first panel than Mr. Munroe (Randall?) usually puts in. Really, the first through third panels aren't that bad. They set up well for an interesting punchline or a funny wordless comic. Also, the situation presented is kind of funny-interesting, with someone faking rock-climbing to impress their friends.
I guess that's all I can say =\.
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Carl again. My own take on the comic was that it was an exceedingly old joke (I recall it being in an old Mad Magazine batman parody...from the 50s. And I'm sure it was old then, too) rather blandly executed. To all those people who suggested that a far better joke would have been to have a guy who goes to rock climbing walls, sets it up so he is standing out from the wall, and thus makes real photos of rock climbers look fake - that is a great idea. That would have been awesome.
Who is impressed by pictures of rock climbing? I went rock climbing some times. I am not a sporty person. I went to like, rock climbing birthday parties when I was like, eight. Why would you be impressed by that facebook picture?
