Heh. I've been doing a rewatch of Buffy and Angel myself. I'm currently near the end of season 5/2. Just finished The Body.
I did always wonder why Jesse was never ever mentioned again, even in passing. He was one of Willow and Xander's best friends. They seem to get over his death really quick and never even think about him again.
As for Luke's strength, the best explanation I can think of is that there is also a big gap in strength between people. Luke was a pretty big guy, so it goes to follow that he's stronger than average.
I agree with you about Darla. However, I can cut them some slack, as by this point she was only meant to recur for a few episodes and be staked. Her whole detailed back story with Angelus and her subsequent appearances on Angel hadn't even been conceived, yet.