So I'm re-watching Buffy and Angel (concurrently, btw)... it's been a while since I saw them both and I'm, yet again, surprised at how good they both were.
So the question stands: why Angel? Why did Whedon give David Boreanaz -- not the best actor on the west coast -- his own show? Why did he take Angel (the character) off of BtVS and separate the two characters (granted, the fork in the road was a natural progression for them, still the chemistry was explosive and, as a writer, I would not want to give up on writing for them together), and why... well, why another show with pretty much the same theme as BtVS?
Anyone have any input on this?
Explosive chemistry? I used to love Bangel back in the day (during seasons 1-3), with it being such an epic romance and all, but even then I noticed that they didn't have the greatest chemistry in the world. It was OK as long as they were making out or when they were in a huge dramatic scene - but just having a conversation... awkward. There were moments when I almost expected her to get bored with the big brooding plank of wood every moment, and him to get annoyed with the tiny fiery brat.

It didn't help that David Boreanaz's acting wasn't the greatest back then (he got so much better later on AtS). But in season 1 of AtS, when he was getting better, the chemistry between him and Buffy when she would guest star, felt so forced. This is one of the reasons why "I Will Remember You" fell flat for me, as soon as they had to play a normal couple instead of epic doom, the sparks weren't there. I thought Angel had much more chemistry with Kate (even if nothing came out of it), and Cordelia, and most of all Darla. Buffy only had great explosive chemistry with Spike. I remember thinking during season 2 "How odd is it that she has more sparks with the guy trying to kill her than with her boyfriend?" (And that was when I wasn't expecting anything to ever come out of it.)
As to why Whedon picked DB to star in his own show... From what I've read, he liked DB's performance in "Amends" and thought he had developed as an actor and could carry his own show.
But I think the actual reasons why him are the following:
Angel wasn't an integral part of the Scooby gang. Willow, Xander and Giles are, and they belong to BtVS. Cordelia also wasn't integral, so she transferred to AtS as well. Angel was primarily Buffy's love interest, and as soon as their relationship ended, there was no reason for him to stick around. And you just had to end the B/A relationship because it had run its course, there was simply nothing more to do with it without destroying the whole "epic doomed romance" aspect that people loved it for in the first place. So either they had to try to somehow turn it into a normal relationship, or end it.
On the other hand, he was quite an interesting character himself, with a rich history, and could be developed in much more complex ways and carry many interesting storylines (and thank god they gave him his own show; he became a much better character on AtS).
Besides, Angel was never one of Whedon's favorite characters - he was more of a creation/favorite character of David Greenwalt. Whedon initially wanted to use Angel for one season (he was billed as a guest star in S1) and didn't have long term plans for him, but as the story developed, they expanded his role. It made sense to let Greenwalt run his own show with Angel as the leading character.