The game isn't Mario kart, nor a "free MK". If you want to play to MK, well just play to MK. Our goal isn't to copy Mario Kart. That would be useless, since you can simply get the original.
What is the goal then? Altough is is not exactly a clone, you can't deny that SuperTuxKart is clearly heavily inspired by Mario Kart. Do you want to be different just for the sake of it? There is nothing wrong in adapting a game concept. It is not even illegal to “steal” ideas.
Personally I think ideas should be valued on how well they have been thought out, not on where they come from. I don't care if feature X is a direct adaption of some other game Y or if it is truly something original (which is unlikely).
Also, do I understand it correctly? Are you opposed in having something like a championship mode in STK?
Personally, I think the Story Mode is quite okay, but I think it is a bit short and also quite easy (for me). In 0.9, I managed to get gold for all Story Mode challenges in about 2 days. But to be fair, I already played STK for a while and finished Story Mode in previous versions a couple of time.
It also certainly fails to provide long-term gameplay, a lot of opportunities are missed: First there are only 4 different grand prix in the whole game by default. At least users can easily add their own grand prix now, this is a plus.
Reverse direction is not used at all which is a huge missed opportunity. Also, most tracks are only used once or twice (they re-appear in a grand prix) and the game modes “time-trial” and “follow the leader” are underrepresented in the Story Mode. I have a few other issues with Story Mode, but I don't want to write more for now. Maybe I will write a whole new thread about this.