So... a couple of thoughts here from my side. (Though I have to admit, it's nearly 2AM here and I'm tired, so didn't read the thread entirely.)
Was discussing the ranking system with theowest in irc some, he referred me to this thread as we got some pretty conclusions (or I at least think we do).
To my experience, most normal users in osu! play this for fun and don't really think their rating through, unless they have a experience in modding/mapping and can underbuild their opinion properly and all - though that is a minority, let's be honest.
People - I admit, I was like that until I started mapping - often just rate songs because they want to continue to the next song or because they want to go do something else. That rating screen is just one extra that pops up (and to some people also annoys).
Therefore, I was thinking that, additionally to liking-disliking (which I think is SURELY enough for people who're not professional - professionals should've modded the map before it got ranked c;) discussed in this thread, the rating system should also be separated.
What do I mean by this?
I think it should not pop up when you get your score chart at the end of your song. Instead, it could for instance be placed with the local scores. The way it seems to me that the people who do vote, are at least slightly more behind their like/dislike, as they put a mere 2 seconds of thinking in it, more than what they would've otherwise.
Also, another question might be: how many times to vote?
After all, people could save their votes up by playing it a lot of times and then get quite some impact by voting all their votes. I don't think anything's wrong with giving people the amount of votes same as the amount of times finishing the song, as long as there is for example a 2 minute buffer in between every vote.
Apart from that. I personally also do agree with separate voting per difficulty~ Though it should be taken in account of one diff got a lot more votes than another, rather than just taking the average of the diffs.