That's because they tried to cram 12 moderately-lowpoly characters into four split screens. Seriously, eight was good enough. All the back four are good for is pounding you with POW Blocks and Blue Shells.
Agreed. However, if it was only 8, I'm sure they'd rubberband a lot more and still get the same awesome items anyway. Also, if you are in first and get killed, you end up in 11th place instead of way behind last place like games only containing 8 racers........I guess that makes it "better" since you're not in last place, am I right?!
Also, I think all they did was add a few more polygons to characters to smooth them out from where they'd typically be viewed, then turned down the saturation some (MK:DD is 50 times as brightly colored as MKWii), then added a Mario Galaxy-esque glow to everyone to show light reflecting off them.
Mario Kart could've looked similar to Brawl or Galaxy if they wanted to put more effort into it. Yeah, I know there's time constraints, it's hard to redo everything to suit the Wii, etc., but still, they could've put some more time into it. Mario Kart's always shown a Nintendo system at its near best. MKWii looks too similar to Double Dash, [darn] near. That's like if Double Dash looked similar to MK64, or if MK64 looked like SMK.