There's a ton that Buffy could have done better. A TON.
I'll start with Caleb, because that's quite possibly her greatest failure. Buffy made zero, and I mean zero, effort to even remotely gauge what Caleb was capable of. She didn't consider for a second that she was being led into a trap, which she was, despite her long, long history of being led into, you know, a trap (to quote Angelus: "you fall for it every time!"). She didn't attempt to scout, she didn't form any sort of battle plan whatsoever beyond "charge!". She just rushed in totally blind, and there's no excuse for that. That's failure number one.
I had no problem whatsoever with her wanting to go back in. You can't just ignore Caleb. However, she wants to essentially replicate her first failure. Buffy has consistently shown over the course of the series that when she's physically outgunned, she has the intelligence to think her way around the problem. We saw that with The Judge and with The Mayor. Why didn't she take a moment to compose a tactical game plan for going back in? No wonder her charges balked. She didn't outline anything she would have done differently. I mean hell, take a rocket launcher again and blow the place up. Try and anticipate how Caleb thinks, and how he'll react. She makes no attempt to do that.
I had no issue with Buffy keeping Spike around and honestly, I don't know why people view it as a problem. He's dedicated, he's powerful and he's loyal.
The...um..."plan" in"Chosen" was a mess. A complete, near suicidal mess. Buffy had no clue what the amulet actually did, and had no actual goal for storming the Hellmouth. What would I have done differently? Let the uber-vamps stream out into the school in the middle of the day, and when they do, have Willow magically collapse the roof of the building, kind of like what Dawn did, but on a larger scale. Or better yet, put in a call to Riley and have him get the military involved. You have thousands upon thousands of suped up vampires streaming out of a totally evacuated town and you don't activate the Air Force? Why? The Turok-Han are exclusively ground soldiers. They have zero air capability. One call to the military, who are fully aware of what's happening in Sunnydale, and the threat of the uber-vamps is kind of negated. Sure, they have some difficult questions to answer, but the threat itself is mitigated. It doesn't make for great television, but ten U.S. fighter jets trump the entire Turok-Han army. Can anyone even make a case against that?
Even if Buffy doesn't want to do that for some reason, turning Angel and, I'm assuming, the forces of W&H away is idiotic. I know the show had to do that because they were committed to Spike as their male lead and it would have been unfair to the plot to not resolve his arc with Buffy, but looking at it tactically, why not have him there? Because Spike is going to be jealous? I get the "second front" idea but really, if Buffy is that concerned about a second front to the battle, she would have called Riley. It made no sense not to have Angel and all his considerably vast resources engaged in the final battle. And if she was hesitant to use W&H it's like you said [MENTION=6758]white avenger[/MENTION], desperate times call for unlikely alliances.
Those are the big ones. If someone thinks I'm wrong logically, and not just wrong in the sense of what a television plot requires please, by all means, test me on it. I could have done much, much better.