--One, terrible use of space. Just terrible. The huge amount of white space makes the comic extremely off balance and irritating to read. Panel 5 could easily have come at the end of a row. Yes, it's true that more time has passed between panel 4 and 5 than between any other panels, but that's not really a problem. If you are going to separate the last one out like that - and you can, that's fine - you need to figure out a nicer way to do it. At the very very least, center it. Probably you should make it a bit wider too, and perhaps taller to match. You could also have the first two panels on one row, the 3rd and 4th below that, and the final panel below that.
A smaller width for this comic would have the additional advantage of letting the caption - which refers, of course, to the entire comic's action - take up the width of the whole comic.
--the towel on the stick figure looks really, really weird. I assume it is a towel, actually, I should probably point out that they also might be pants.
--Here, I'll switch to saying something nice. People have pointed out that it appears the cut-off #4 on the first panel's list says "bee eating contest." I find this line quite funny. I think if I had been editing this comic I would have said to make Bee Eating Contest the first problem, and get into the funny sorts of situations that would bring up. Snake bite is irrelevant enough to be weird but not inherently funny enough to make for a good comic.
--It's another lame "what if a human mind was operated like a computer could be?" scenario. And the answer is: "it would lead to hilarious consequences, all of which would be hilarious!"
--dialog in the last panel. Both of them. There's no shock in the woman's voice - her question a about his preparedness isn't even until her second sentence - and her first sentence ("I'm here to pick you up") is one that in real life would be so obvious as to not need stating. Even "Wait - why aren't you dressed for our date?" would make more sense.
As to his dialog, "By LD 50" is needless nerdery. Toss it.
--Lastly, the art details. First, check out the classic example of xkcd floaty-head syndrome in panel 2 (did we ever come up with a name for this symptom? I have forgotten). But more importantly, notice that the hair changes between panels 3 and 4. Given the towel, the fact that he's preparing for a date, and the general mess that his hair is in, I assumed, as did many people, that the hair simply had been wet, perhaps in some kind of a shower, and then dried as the time went on. That's fine: Standard, presumably dry, xkcd hair is just to have nothing. By switching to that, the comic was, I assume, trying to show that time had passed. Good! That's a clever way to do so, without calling attention to it too much. I was almost shocked, actually, given how badly Randall usually does the telling and not the showing (see, for example, panel 5 of this very comic).
And then, as you probably know, something changed. Namely, the comic changed. Randall went and made some edits and reposted the "fixed" comic, never, as far as I know, admitting he has made a mistake (this has happened a few times with typos and such, usually updated on the sly and never admitting that things were any other way). Luckily, our own Ann Apolis saved a copy of the original that you can see above. The new one is, of course, on the xkcd site linked to through the image above.
Obviously, having to go back and update any comic is not a good sign - it's a sign of hasty work, not checked before it was published. But in this case, I think there's an even weirder problem, which is that the comic was better before.
Think about it - there are, in the revised comic, two options:
--Lots of time passed, and his hair is just always like that
(this is odd because almost no male characters in xkcd have ever had hair, and when they did, it was for a specific and clear reason, usually to identify them as unique)
--No time passed, and his hair is still wet
(this is odd, because the humor of the comic is supposed to come from the fact that he has wasted a lot of time on doing the wrong sort of research)
See, when I say things like "there are things about xkcd that are objectively wrong," this is what I mean. The way he's constructed his comic has clear and identifiable problems. It's not like it's impossible for him to do it right - after all, he did it right first, before he changed it. Yes, some people were confused by "what happened to his hair??" but those people should have thought harder.