I think it works best to judge a race's difficulty by how hard it makes you want to shit your shorts when you're standing on the start line. Obviously many things such as the competition, your racing style, your expectations for the race, etc. have an effect on this, but it kinda gives you a way to compare different types of pain too. Because obviously there's not much worse than 550 to 650m into an 800, but 5k pain is almost as bad and it lasts forever more or less. So I figure whichever you look forward to experiencing the least is the one that's harder.
So for me while running an 800, I want to win, I want a fast time to go with it, and I've invested a lot in my training to get there, so I don't want to f*** it up. My strategy is pretty much to run the first lap as fast as I can while leaving just enough in the tank to finish the race. If I do that I know it's not going to be a fun second lap but if I don't, I'll be pissed at least until my next race. This means I'm not looking forward to my task when I'm standing at the start of an 800.
In a 5000 I know it's going to suck after like 8 laps, but who cares, it's a 5000, I'm just going to find someone I think will run as fast as I can run and sit on their ass and not really worry if it's going to be the perfect race for me in the end. It just doesn't scare me as much as an 800.
Based on the threads around here marathons seem to work people's nerves more than anything else and I can understand why, even if it's not my cup of tea. The shorts-shitting-o-meter says marathons are the toughest race.
As for the 600, I run one every few weeks to gauge where I'm at. I guess it's hard, but I just expect to tie up around 350m. I never get the combination of chest and leg pain like the 800 and up.