International Gateways: Generally domestic travel to the gateway is included, but sometimes not - check your specific plan/award. For example, British Airways' Concorde awards are for travel from New York only; you'd have to get yourself to New York.

Stopovers: If you use an "open jaws" ticket, that generally counts as your allowed stopover.

United vs. Delta: I believe that United and Delta only allow cross-airline mileage accrual, not cash-in (anybody confirm?). And I'm pretty certain you can't combine miles from UA and DL towards a single award. That makes this alliance less useful than the AA/US one.