You should make a restriction on how long a song a person requests can be. I say 30 minutes is a good cutoff, since you probably don't want to sit for much longer than that to tier a song :P
This. Not the exact amount, but the need for a time filter. I really didn't like sitting through hour-long songs several times to figure out what the tiers would be. No offense to metalhead, they're great songs, but it's kinda boring sitting through the same songs several times to figure out if it's all devils or all lower difficulties.