Love Firefly and Serenity. And since I can't remember if it was mentioned on the commentary (I listened to it a few weeks ago), Morena Baccarin also admitted that Inara's story arc involved her having a terminal illness, which was why she was on the ship in the first place. Would've loved to see that arc play out - the pain, the angst... Joss would've milked that one for all it's worth!
Also, given that Joss cast Alan Tudyk in Dollhouse and Alan has many times publicly lamented Wash's death in the movie, I don't think any speculation that there was a falling out between them, or that Tudyk wanted to leave the franchise (what franchise?!) is anything other than speculation.
And for the record, the second half of season 2 and season 3 of Buffy are the best, but Season 4 has "Hush", "Who Are You?" and (most of all) "Restless" (my favourite episode of all) ... season 5 has "The Body".... and season 6 has "Once More, With Feeling" and "Normal Again". Those episodes (almost all written by Joss) are fascinating episodes of television, all in some way pushing the boundaries of what we're used to seeing, and they all work really well. (Whether you could watch any of them without prior knowledge, I'm not sure. Probably not. But they're fun.)