I'm gonna critique Mario Kart mode here, seeing as it's pretty much the only mode I play at the moment. It's already obvious Single Player is incomplete and buggy, and I'm not gonna drive that point home.
Anyway, for the most part...I love it. It totally blew me away. I played Mario Kart DS for years and years, and I was amazed to find how similar the controls felt. It took almost no getting used to. And I loved all the little changes that added up to make it feel like the real thing. Best of all, it's really, really easy to design new tracks...Rainbow Road took only a few hours of work in total.
This is, quite simply, the gold standard for mods. It does everything that a good mod should do--dramatically change gameplay while still working within the appropriate constraints of the engine and graphics style.
With all the mushy stuff out of the way, it's time for the nitpicking. I have several complaints about minor issues that I'd like to see resolved in the next version.
1. Unlike in real Mario Kart, item frequency doesn't seem to be weighted by your position. I often snag a Mega Mushroom or a Lightning Bolt while I'm in first place, or get a bunch of green shells and bananas when I'm in last. Weighting item frequency is a crucial part of the game, as it constitutes a negative feedback loop which gives the laggers a fighting chance and makes the race more interesting.
2. Shrunken-down characters shouldn't also have their ramp-sector threshold shrunken. In vanilla SRB2 this is perfectly fine, and even desirable, but in Mario Kart mode the players often have to drive up tons of steps, and sometimes the steps are slightly too high for shrunken characters to reach. This means that an already-handicapped player can get stuck for several seconds. I know of at least two levels where this is an issue: Peach's Castle, and my own Rainbow Road. This may speak more to poor level design of the tracks than anything else, but perhaps this is still something you want to think about.
3. In a lot of the included stages, it's way too easy to miss a Star Post, which can instantly kill your chances in the race. Again, this is far more bad level design than bad mod design, but perhaps those stages should be tinkered with.
4. Is it possible to throw Green Shells behind you? If it is, I haven't been able to do it. That's a positively vital function of Green Shells, seeing as they're supposed to be used more frequently by the leaders (disregarding my previous statement about this mod's lack of item weighting).