I think Buffy ended the way it needed to end, at the time, although to me the real series finale is the end of Season 5. Season 6 was just moronic, and made me want to slap Joss when I finally did meet him for real. 7 was decent, and you could tell they were trying to get it back, but the heart had just gone out of it. I do love what the real finale set up in the universe, with so many damn slayers walking around. It would've made a perfect Faith TV show if UPN wasn't made up of fucktards.
As for Angel, again, brilliant show, Season 4 is my favorite season of any TV show ever. However, I disagree that the killing of Fred was "unforgivable." It was necessary, as Fred was even more of a background chump than Xander. Illyria, on the other hand, was interesting as hell and the stuff between her and Wesley would have carried another full season on its own.
And hey, they didn't necessarily all die. The only confirmed death is Wesley. With Spike, Angel, and especially Illyria on their side they might have made it out.