I think Whedon's projects lend themselves better to long-term serial-form projects than to finite series. Once he sees the end zone, he starts strutting, and there's always going to be a Don Beebe right behind him. Buffy died after "Tabula Rasa" (or, more accurately, when Joss got sucked into Firefly), Angel's last season was a maudlin morass with moments, and Serenity definitely had the air of breaking a few toys on the way out of the room (and then there's Whedon's run on Astonishing X-Men, in which he gets to ice one of the characters most inspirational to his collective mythos). I think it's a philosophical and narrative point of disconnect that I'll always have with him in that while it's always hammered into writers to "kill your darlings," death is not always automatically dramatic, and the manner and timing in which you do it is not arbitrary. Deaths in the tail end of Buffy and Serenity were especially unresolved, to which he'd say not necessarily uncorrectly that that's the nature of death, but that's not the best nature of fiction. In this I think there needs to be that level of artifice and symmetry and suitability or else we're disturbed and disquieted and not in a good way.

A couple of notes: I'm not counting the comics of the TV series because they've gone from interesting hypotheticals to extended train wrecks the longer they're allowed to continue, and I've never seen more than an episode (if that) a time of Roseanne, but I understand something of the charlie-foxtrot it became in its later years; I'm not sure who gets the blame in that regard, but I'm willing to saddle that on her and not him, if only because I'm willing to believe a second-generation TV writer has less clout than a one-named TV diva.