Initially, I liked how the characters would have ordinary conversations- strips where they're just hanging around "shooting the shit" are some of the most charming.
But QC actually does "Serious Business" storylines very well- Faye's revelation around #500, the Marten/Dora split, and other storylines were actually handled very well. New characters are added to the strip semi-organically, and they're entertaining. Many of them avoid the "everyone's the same" syndrome that he suffered from for a while there. I really liked how he'd have Marten and Dora have the same fight
every few months or so- that's actually how real couples fight
The characters have nicely-varied personae- Marten is overly-passive and apologistic (though Jeph has that same trait, and seems to defend it stridently). Faye is obnoxious and rude, which seemed cute in earlier strips, but as the art & comedy style got a bit more realistic, it started getting uglier and uglier. Dora acts confident, but is actually extremely insecure and hypocritical. Raven was an idiot, Sven was a man-whore, Hannelore was a mess of psychoses, etc.
I even appreciated the "Indie Rock" obsession, as you didn't even need to understand it to "get" the jokes- the characters were into it, and the jokes revolved more around their fandom than actually requiring you to know about the bands. I kind of missed it once Jeph dropped that element.
There are some noticeable flaws, however- I feel that Jeph's art style has actually gotten more stiff over time, with very thing line width making things very dull to look at (the strips around #500 are visually the most interesting). Some of the new characters completely overshadow prior, interesting ones, and Jeph often lets a new favorite run away with the strip. Every year or so, QC essentially completely reinvents itself around one new character, making them a massive central element (Hannelore, Marigold and Claire have all gotten this treatment). There's a LOT of "Chuck Cunningham Syndrome" as a result (poor Raven & Penelope). Similarly, Jeph over-uses the "Crazy Girl" archetype, with most of them having mental issues, while the men are much more passive, together, "boring" characters (Steve, Amir, Wil, etc.). Jeph can also trail along with nothing storylines for way too long, leaving large chunks of the strip pretty dull.