I don't know if the odds of a subreddit succeeding are more or less than for an LJ comm, but I can tell you that there are a LOT of completely empty subreddits.

The fact that reddit doesn't really have an equivalent of a "personal journal" helps it along, though - you either pick an appropriate subreddit, or you don't post, there's no "my friends would like this more than the comm" option unless your friends have their own comm.