I typed out an email and thought I'd just leave it here as it answers your question:
Hi, I was thinking about something like this after reading Jaron Lanier's Who Owns the Future (which I recommend simply for his view on how a capitalist economy could subsist on micropayments, if you haven't read it).
The world needs something like this, if only because the concept of post-payment, if successful, would align expenditure and value to a degree that is impossible if you've paid for a good before you can consume it. If this got big, it would change the quality of music that is produced as musicians would strive to make music that lasts. It requires a very moral user base, but I think you can find one.
I saw in another comment that you might use analytics as a future premium feature. If you think of the donor's psychology, tipping an artist comes from a desire to benefit the person who has provided them with an enjoyable experience, but also as an indication that of all the artist's material, that song/album/video is the direction that the band should continue to go in. If a donor thinks he can influence a band's decisions, then I am convinced that he would be likely to give more (remember, we're dealing with a fairly well-educated morally inclined user base. Not all people think this way, but they do). If analytics is locked behind a paywall, then donors might not feel the same incentive to give. This is obviously problematic, since you seem to be leaning freemium rather than charging fees as a source of revenue, and need to think of good premium features that the free user base won't miss.
If you truly view this as tipping, as a means to correct the fact that musicians receive far too little of their music's sales, then the payment model makes sens. If users start to use your website as a moral justification to steal music (as I certainly would) then you're effectively cutting into the labels' revenue. Though I applaud that initially, if this starts to gain scale, I would consider allowing users to give the labels a small percentage of their donation, as the labels do play an important role in producing, marketing… though not as big a role as their current compensation would suggest. I doubt that labels would ever accept these donations, as it would be an admission to the inadequacy of today's music distribution model, but you would gain a reputation of being a fair player rather than a pirate.
Though it sounds like this is already the case, I would highlight to your users that what they are giving is a pledge: it's not guaranteed that artists will agree to accept these donations. The pledging mentality, like with kickstarter and co, makes people give more as expected expenditure = pledge x probability of acceptance. An economically inclined user might give $15 instead of $10 if he only thinks that there's a 2/3 chance of that
Finally, you should allow for users to supply and request content associated with artists, tracks, albums… Hey, I'm looking for guitar tabs for this song and am willing to pay x for it. First person pays the fee, other downloaders can tip the guy who made the tabs if they choose. Or, I'll make tabs for this song for $20, people can leave little tips, and once it hits $20, it gets sent over to them and is available to the rest of the community. If there's too much content being generated, use a reputation system to only show the best and keep the UI clean.
I may be thinking a little too niche because I play guitar, but this could extend into all sorts of other spaces like merchandise, though competing with ebay/amazon on that front probably isn't where you want to go with this. If you did get into selling physical goods, you could keep it pretty indie by only allowing user created content (a poster a user drew). Because you would be creating a market that doesn't really exist, you would be more justified in charging small fees for user to user interactions than for user to artist tips.
I'll stop rambling.
I look forward to following your progress.