In actuality, Willow doesn't drain Dawn, but Kennedy and Anya, so there's no supporting evidence that Dawn is the most powerful being in the room here. Dawn and Kennedy are knocked down when Willow's eyes first go dark and some powerful magic knocks Kennedy and Dawn over, but that is not the draining exchange. That happens later; Dawn's still lying on the floor from being knocked over earlier when Willow zaps Anya and Kennedy for power to open the portal. Willow later explains to Kennedy, "It's just, you were the most powerful person nearby, and—well, that's—that's how it works. That's how I work." Anya was also the most powerful person nearby. Xander and Dawn are close by, but Dawn is lying battered on the floor--is her Key power not relevant to opening a portal then?
It seems important to note that Willow is specifically attempting to open a portal and Dawn is not the "key" to opening it. Once again, the writers treat Dawn as normal and human in a situation where her being the Key to opening a portal would be a perfect plot device. For Willow to not even consider that Dawn's power could open a portal, for Dawn to not even consider that she might be able to open a portal, it strains credulity. Unless all the characters think Dawn isn't the Key anymore.
The writers treat Dawn as normal again in Potential when Andrew describes Dawn becoming a Slayer as being "plucked from an ordinary life." Xander later relates to Dawn with this exchange:
Seven years, Dawn. Working with the slayer. Seeing my friends get more and more powerful. A witch. A demon. Hell, I could fit Oz in my shaving kit, but come a full moon, he had a wolfy mojo not to be messed with. Powerful. All of them. And I'm the guy who fixes the windows.
Well, you had that sexy army training for a while, and—and the windows really did need fixing.
I saw what you did last night.
Yeah, I— (shakes her head embarrassed) I guess I kinda lost my head when I thought I was the slayer.
You thought you were all special. Miss Sunnydale 2003. And the minute you found out you weren't, you handed the crown to Amanda without a moment's pause. You gave her your power.
(shrugs) The power wasn't mine.
They'll never know how tough it is, Dawnie, to be the one who isn't chosen. To live so near to the spotlight and never step in it. But I know. I see more than anybody realizes because nobody's watching me. I saw you last night. I see you working here today. You're not special. You're extraordinary.
(from the couch where she sits, injured) What are you gonna do?
What I do best.
Spike slams the demon's head into the wall, but it turns the tables on him, throwing him so hard into the ceiling that he breaks through it and lands on the floor upstairs. The demon breaks through the closed French doors, smashing glass as he leaves the house. Willow sits up where she'd fallen. Xander also tries to stand. Principal Wood helps Anya stand.
(to Kennedy, who's inspecting her bloody hand) Are you injured?
Just hurt. Don't make a case out of it. (to Xander and Willow) How 'bout you?
I think we'll live.
(looks at the ceiling) Except maybe Spike.
Is getting thrown through the ceiling what he does best?
(sighs) I'll go check on him.
Dawn is the first and only one to be concerned about Spike's well-being. She cares. But it gets more interesting.
Dawn leaves to go check on him, then in the next Scooby scene she's back with Xander, Willow and Kennedy researching, explaining how "once Buffy left [the book]" went blank. In the middle of discussion about what kind of spell they need to do to get Buffy back, exchange the demon and use a catalyst, Kennedy asks if it "matters if it's dead or alive", Spike shows up and says, "I vote dead," explaining how he's gonna go after the demon.
As Kennedy is giving Spike a tough time, expressing doubt 'cause he "can barely stand", the camera shows Dawn looking very uncomfortable and avoiding eye contact by looking down (28:25).
Something happened in between Dawn going to check on Spike and when him coming downstairs, something that made her uncomfortable and want to avoid him. When she says she's going to check on him, she's fairly upbeat and she's certainly not upset. What happened? Did she talk to Spike? Did she become overwhelmed at the idea that she still cares about him when she's been determined to act distant?
Anyone care to fill in the Spike-and-Dawn blank?