First, I would accept unsolicited queries. I feel like there is so much out there and maybe I'd get lucky enough to snag a gem! So, yeah, I'd want to have the chance to find it.
Second, I would ask for every query to include the first chapter of their book. Now, I say first chapter because it has been my experience that MANY good books I've read start off kind of slow, and maybe take several pages to get going. Not that I think that is good or anything I just think that I would want more time to make a decision. That said, if the query and first page or two completely disinterested me or was incredibly awful, I would stop reading.
I would have a polite standard rejection for queries. I actually don't mind standards. But, I believe in constructive criticism, I don't believe in flat out dashing someones hopes, so I know it would be a very thoughtful rejection query, LOL! I am absolutely of the opinion that just because I think something sucks, doesn't mean it actually does to everyone else.
If I ask to read more, I'd probably ask for a full. But in the rare case I just needed a little longer to see if I was actually interested, I'd ask for a few more chapters. But, I think I would probably know after reading one. In those cases I would personally respond. I feel like if you get an authors hopes up and request a partial/full, then reject, they should know why. If it just didn't capture my attention enough, I'd say so. If I thought the plot was weak, I'd say so. If something didn't make sense I'd say so. I just feel like it would be my responsibility as someone who has elicited hope in this person to tell them why I'm rejecting it. Maybe my words or reasons would help that person to make the book the best it could be. But that's just me.