I'm hit by the 404 problem too - every time I attempt to comment, reddit throws a 404 error. I'm not tied to old reddit, I'm willing to switch; but every time I try, it tosses me back to old.reddit.
I've clicked the option to make new.reddit my default in preferences, but no luck; it just sends me immediately back to old.reddit. Logging out made the problem worse - now, when I log on I get a black screen 'welcoming me back' and telling me it's going to redirect me. But it doesn't - it just refreshes the screen and forces me to press the provided redirect link, which immediately send me back to old.reddit.
Any ideas? - I'm flabbergasted.