Characters. Really all of the above, but that isn't an option. The OoTS characters and villains (heck, even the supporting characters) should be the new Iconics whenever 4th edition rolls around.
Humor mostly (not always ROFLMAO type of funny, but if I can at least crack a grin it's all good), with a 2nd place tie for Characters (Roy and Belkar) and 'cause it's D&D in 'toon form.
You know, a lot of people complain about that happening to a lot of webcomics.
But, if it's the inevitable evolution of a webcomic, if it's going to survive. Take a look at any long-running webcomic (Something Positive, MegaTokyo and Apple Geeks are a few good examples) and you'll see the same thing... The comic begins as a series of fairly unrelated jokes, slowly develops a story line, and occasionally pops back into the random jokes, just for fun.
It's hard to keep a three-days-per-week web comic going on jokes alone. You'd use up all your good jokes real fast.