Do all dieties acknowledge general "rescue me" prayers?
Almost none do, in fact. Elyvilon can protect you if you pray, and The Shining One will occasionally nullify an otherwise-fatal attack. Zin has Sanctuary at high enough piety. I believe that's it (Yred can damage enemies for hurting you, which I suppose is similar). Gods are on the whole more pro-active than reactive in Crawl ... and unlike in Nethack, they expect you to be able to stay alive on your own.
Do any of them require sacking at the altar?
No. There are a few who accept sacrifices only at an altar (Sif Muna and one of the good gods IIRC) but the majority accept sacrifices with a simple prayer over the object of sacrifice. None of the few who accept altar-only sacrifices require them to raise piety and I nearly forgot they even exist.
I never got god-gifts inNH, and I have only seen Nemelex hand out cards (a whole other topic for discussion). do all gods grant gifts, and, if so, do they require differnt methods of making this happen, and what determines what you get?
Most give out some gifts, which you get simply by raising piety. In most cases, this means killing stuff or killing stuff and then sacrificing corpses.
Xom is a bastard, why would anyone choose to follow that psychopath?
He makes games interesting. Crawl doesn't try terribly hard to make all routes to victory the same difficulty, after all. If you actually survive he tends to turn your character into a highly-mutated freak, but similarly if you survive the mutations tend to be helpful in the end. I think he's generally more beneficial than harmful if you're keeping him interested ... but when he decides to be mean, yes, he usually really messes you up.
I killed off my SENm somewhere in Vaults I believe. Silly overconfidence as usual, coupled with a disastrous sticky flame incident that mangled my scroll stock. I managed to score a lajatang of speed off of Agnes before I died though.
The only opinion one should care about is one's own.
We did. You did. Yes we can. No.