Personally I can't resist a couple of fanwanks there. Anya is lecturing the potentials at one point, and tells them it is possible to stake an uber vamp, but it's just a little harder than they first thought, so one can postulate specialized training after more information is available. The first ubie was a surprise to them, and they didn't really know how the mechanics of the turok han worked.
I once heard a fanwank that the original Turok-Han was some kind of supreme warrior amongst the population, so he was sent out first, and the rest were weaker.
As for Season 7, I think the main problems, aside from some plot holes and leaps in logic, was just that there were too many missed opportunities.
Firstly, you have...well, the First. After a while, the First was really just assuming a few roles, and it got tiresome seeing it as Buffy. I understand that there are some restrictions because of the availability of actors, but I wish we'd gotten to see the First as more of a "who's who" of old characters - at least, more than the couple of times we did (I'm thinking of the ending to
Lessons). I also think the First, as a villain whose main power is psychological, could have been utilized to torment the heroes even more.
I also thought that there were some missed opportunities when it came to Spike as well. At the beginning of the season, I
loved that his soul had (apparently, at the time) driven him crazy, because I was terrified at the end of Season 6 that it was going to be nothing but Angel-redux. However, I always felt that Spike mellowing out as soon as he was out of the Sunnydale High basement was a poor choice. I don't mind the First tormenting him and therefore adding to his insanity, but I kind of wish his fight back to sanity would've been a bit more substantial. It would've made things a
lot more interesting in Season 7, and it would have made Spike's story arc (and his pivotal moments in
Get it Done and
Chosen) that much more powerful.
And Giles. Oh, Giles. Not that I want you to be dead, but I really wish that the writers had stuck to their guns and not changed their minds about you as soon as all of fandom figured out that you were actually dead.
There's more, but I have to rack my brain to remember. It's been a long time since I've watched any episode from Season 7...