Delta has impressed me lately. I may make the switch and just deal with the extra haul to JFK which is about 20 minutes more for me if United can't or won't improve their lousy, outdated aircraft for international flights to Europe.
United does fly nicely outfitted aircraft to Asia and Australia but I don't get out that way nearly as much.
Of the big three legacy carriers, Delta is currently the best combination of price and reliability. You know they want a bit of extra money for the slightly nicer experiences, and you know what you're going to get out of it.
For international travel, though, I've been starting to avoid US carriers entirely in favor of flying the other country's airline (which is something you can do when living near a major international hub -- SFO, in my case). Often it's another small step up, and sometimes it's a big step up, compared to the US airlines.
You're right though, airlines like British Airways and Virgin Atlantic are quite a bit more nice. United treats me pretty well as I have the highest status, however.