Because it's boring...
Buffy was mostly an action-adventure show and season 7 is mostly soap opera territory. Buffy, I don't care about your deep personality problems and your troubling relationships and the rest of that nonsense, just go and slay something already!
Uh...OK, I'll give you an example which I believe sums up precisely why season 7 sucks.
In season 2, Buffy grabs a stolen rocket launcher, walks into a mall full of people and with a criminally bad pun, blows up a no-good demon into a million little pieces! Because she's THE SLAYER, an unstoppable goddess of mass destruction. And she's beautiful and she's glorious.
In season 7 Buffy takes a rocket launcher again and...NOTHING happens, we get Spike running around with it like the wanker he is. She doesn't even shoot the damn thing.
Do you get it now? Do you understand the difference?
But mostly because it doesn't make sense!
Now there are potentials, they waste precious screen time and honestly, why would we even care about them, but that's not the biggest problem. You know what is? They are pointless, utterly useless. All Buffy needed to do was to strap a shiny whatever that thing is onto Spike's bare chest and drop kick him into a hole in High School's basement. Done, The First is defeated, congratulations, go buy yourself some nice shoes.
And why do they even live in Buffy's house, get a hotel you tramps, it's not like you are special, there are thousands of potentials around the world, thousands and new ones are produced or whatever on a daily basis, sorry Giles, you're officially the worst watcher ever, The First can't kill them all, it's impossible. Uh!
The ending? It's disgusting, it's insulting. So Buffy, having first hand experience of how unbearably shitty, horrible and painful Slayer's life is, being fully aware that the source of Slayer's powers is basically a demonic rape decides to drop that crap onto thousands of...children? Seriously? Potentials are mostly underage, the youngest Slayers were 12 for god's sake.
Can somebody slap some sense into that irresponsible, heartless, dumb little bitch?
Maybe her friends? No, they are all like "Oh jolly good, what a bloody brilliant plan it is!". Have all of them just lost their bloody minds?
And when people say that's she's sharing her power they make 3 lies in one sentence - it's not her power, it's not power - it's a curse and she's not exactly sharing it.
Season 7 is rubbish.