You raise many good points here Rudy. But keep in mind that our main developers are just 2 people, who also do all this stuff for free in their spare time. Hand-holding every contributor would be very time-consuming, and honestly I would rather have them spend time on developing the game than try to help every Blender beginner to become a Blender pro. In order to help SuperTuxKart people have to be able to do some research themselves, or they can become a burden on the developers. But they are here, on the mailing list and on IRC, and gladly answers questions from people that shows they want to help STK.
Furthermore, giving constructive feedback can be very hard at times - how to tell people in a subtle way that their work needs to be improved, while not disheartening them? I'm sure Auria and hiker love to see new content by new people. But sometimes it's simply not good enough, and then people have to be able to take it when they are told so. hiker and Auria don't mean any harm by it, but sometimes they just have to turn contributions down.
As an example, I can tell that when I started roaming the SuperTuxKart mailing list, I didn't have too much to offer other than my enthusiasm and (bad) ideas. I got quick, serious responses and soon started to feel at home. I have many times offered some of my music to the game, and in a couple instances, they were accepted. But most of the time, I was told in sensible ways that it wasn't good enough, with some short reasons as of why. I have to admit I got a little disheartened, but the reasons were sensible enough and I knew that I had tough competition from others. Fast forward to this day, and I still haven't got a lot of music in the game - but I'm happy for the little difference I can make - not mad even though I don't always agree with things. And sometimes, I've understood a while later that my ideas and contributions were indeed inferior - if they had done it my way all the time, the result would probably be horrifying!

I'm very glad to see all the work you, asciimonster and everyone else have put into this. And I certainly don't want anyone to feel that they're not appreciated. But in order to get a good game, everyone and their hamster can't get their stuff into the game. Sometimes it's not because of quality, but because of licensing. That's sad, but it has to be this way. If STK only distributed on its own website, with no Debian, Fedora etc packages, I'm pretty sure STK would not be half as popular, and many contributors would not be interested in helping a non-free (as in freedom) game. So we just have to accept this, and try to confine with the documented guidelines.
P.S: Those who follow the commit logs may have seen how much time hiker and Auria have spent on cleaning up karts and tracks, sometimes improving and occasionally re-exporting all of the game's assets when needed. They do a lot to ensure that things are working correctly, if the contribution is good enough.