While I'll agree that the F-Zero series seems to get a bit of the shaft, I don't see why DK was. It was one of the few series with two stages, it got a new character, and got a fairly large amount of trophies. I'll level with you that Diddy should have been in since Melee, but that's a problem with Melee, rather than Brawl. The only thing I can think of where it really got shafted in was in the AT department, since the Mario series stole its AT (along with the Yoshi series AT) but that's fairly minor.
In regards to Ridley, his size doesn't matter. The fact that he can fly doesn't matter. Sakurai's actually stated that Ridley could be possible. Ridley's popularity is second to no other first party character who hasn't gotten in yet. His popularity was even acknowleged by the fact that he got two boss fights instead of one. While there is stiill a chance that he may not get in, there's no real physical limitation to his problem, and there is virtually no problem in terms of fan demand.
Also a funny thing about music. Series that were more focused on older games/GBA games seemed to get more remixes by default, since I guess Sakurai didn't want GBA songs getting in. Kirby for the most part has older music, with a few exceptions, but on the whole needs a fair amount of remixes to get its musical act together. On the other hand, Metroid has a fair deal of perfectly good modern music from the Metroid Prime series, and despite that, still got a butt load of remixes.
And in regards to those two remixes posted, that's just how the original song kind of sounded like. It really can't be helped. Really, the fact that one song is more low key than another doesn't mean that it had less attention paid to it. (Incidently, a lot of people seemed to complain about that remix of Zero Two)