Rock Band 2 improved on Rock Band in many ways. Rock Band 3 it was nice that people could join at any time, but it felt like in many ways it was not as good as the first 2 to me. Then came the point were new DLC songs would not work in RB2 because of the format change.
3. To be fair Harmonix tried hard to use standard USB. At least on the PS2 and PS3. Certain other console companies did not want this. Activision was the one that didn't want to be compatible and did everything they legally could to stop compatibility. They blocked Harmonix from publishing a patch to make Rock Band compatible with the Guitar Hero 3 controller. In 2010 Bobby Kotick of Activision said that it was a mistake that they didn't talk to Harmonix after acquiring the Guitar Hero brand.
4. The joys of licensing. That being said, I thought I read about them making sure they were licensing all the tracks in RB2 so they would be exportable to newer versions of Rock Band. Obviously something went wrong. One of the things I liked about RB was that you could export most of the songs, unlike Guitar Hero.
5. I think the songs available had a lot to do with licensing again. You want a big name band's less known song, $0.99 a song. You want one of their top songs, $2.99 a song. They had to balance number of songs, with licensing costs and the selling price of the game. This was true of DLC as well. Even the devil (Apple) has to deal with this, and they have a much stronger bargaining position.