As an amateur (meaning I don't get paid) writer, there is a constant push to "show don't tell" when writing a story. Mostly that means don't SAY someone is angry, have that person do or say things that SHOWS that they're angry.
Which is where webcomics have an advantage: They have art to SHOW such things. Sadly, comics that successfully show rather than tell is a small group indeed.
So what goes wrong? Well, it's difficult to really say, but I think a lot of it comes from the fact that the artist has a story they want to tell, and they want to get it out there as quickly as possible. Pictures, while they can deliver a story, take MUCH more effort than banging away at a keyboard for five minutes.
Avoiding this is the topic of the day, so let's begin.
First of all, don't force feed the reader character descriptions. During my review process, I NEVER read the cast page for a comic until after I've finished the archives. The reason is so that I can see if the character I discovered in the comic matches the character in the description without my perspective being poisoned by the description first. The artist should be able to make the character live WITHOUT posting the description at all, let alone as a strip in the comic. I'm looking at you Pure. Comics are ART primarily, so let the art take the burden and have them move and act in such a way to help describe their character.
This isn't to say you shouldn't find a way to describe less than obvious character points. The Meek's Pinter is actually a good example of this. Upon his return to the story after initial contact with the green haired, and very naked, protagonist, he doesn't remember her. It's not obvious why either, though hints are there (he drinks too damn much and forgets things), but it should probably be stated SOMEWHERE in the comic as to why that happens. Of course, it might happen later, in which case that's fine, but at the moment it's just confusing.
Next is the backstory of the comic, which for a detailed story comic can be pretty damn important. However, writing a five page text crawl describing everything for a COMIC is pretty silly, and is exactly what turned me off of Daniella Dark. Working this kind of thing into a comic is actually pretty difficult and requires some tricky writing, however, so I can't fault Daniella too much (though the character description block was PART of the backstory, which made the whole thing worse).
leveL likewise had the same problem, though it put off most of it until about halfway through the comic and did so in a way that was teaching the CHARACTERS the backstory rather than just presenting it to the READER. Angels 2200 actually pulls this off vary well by spreading out the basic facts of the story's backstory throughout the comic via personal accounts and such without it becoming a full on history lesson. In fact, I never read the comic's "briefing" page, yet had more than enough information to figure out the background from just the comic itself.
And finally, let the reader piece the story together, or if there must be a leap of logic, then try to present it in a way that makes the most sense. That last part is actually kind of hard to pull off, actually, perhaps almost impossible. I'm not talking about recapping events, though it is a related phenomenon, I'm talking about drawing all the various pieces of the story together and telling the reader WHAT has been going on the whole time. This is especially prevalent in epic comics, and as the story goes on often only gets worse.
Surprisingly, the guiltiest party I can pick out is a comic I read, Waspi Square, which will spend a week just summarizing what has happened, conjecturing on the future, and even doing new revelations in bouncing dialog between two characters. It seems natural, but it's also not that interesting and takes a while to read through it. The upside is that the story is actually interesting and while the comic doesn't show as much as tell it, it's at least interesting. I would also like to point out when the comic does show, it shows it very, very well.
Waspi also shows that even if you ignore the "show don't tell" aspect of story telling, doing the telling WELL can be just as effective. So show vs tell almost always comes down to a draw, if the writer and/or artist is up to the task of pulling it off. That doesn't happen often, but when it does, you're looking at something special.
Well enough of this, see you next time kiddies.