...sorry, I was entrangled by the awexome news that there's going to be a Strong Bad game on WiiWare.
Okay, back to topic.
Do not miss the no-fullscreen-in-single-player-mode. Sure, it was cool seeing the track layout and racer positions at a glance, plus it's entertaining to see Yoshi's tongue sticking out or Bowser with mouth wide open lasting about ten seconds. I don't recall it being too helpful, though. And once I saw Mario Kart 64 where everything was bigger and it was full-screen... yeah, I want fullscreen.
Coins... I never knew what they did back in Super Mario Kart until I was told. And it's a cool idea to help reward people with the skill of driving over them. But again, since I thought they were useless beforehand, I don't miss coins too much. We've got items (which seem more accurate in later titles) to compensate.
Accumulative speed is cool. But I never noticed because I never got used to power slides (very "slippery" compared to Mario Kart 64), so I always went off track.
Feather is a cool item that I do miss. I don't remember using it much, but it sure is helpful to create your own shortcuts by jumping over sections of track.
Computer superpowers... that I'm glad to see gone. Always irritated me that I couldn't do the same powers the other racers did. I wanted to lay eggs, darn it. I've always accused the racers in the Mario Kart series as cheating (although in Mario Kart 64 during my prime I found they were the stupidest racers on earth, I consistently got 1st on every track), but it's a big no-no for your opponents to do stuff that you can't.
Number 7 is just plain confusing. I think what it's trying to say (how on earth could they screw this up) is that in Super Mario Kart, each track is 5 laps instead of 3 laps. So yeah, when coming back to SMK, I was surprised at how much longer it took to get through the tracks. And I believe what they say later is that due to extra laps, getting hit by a red shell won't kill your chances of winning because by that time there will be a huge gap between 1st and 4th place.
Disappearing barricades... hey, I liked those. Yeah, gave the ghost tracks unpredictability, if a wall's there then it's a saving grace if you bump into it, but if it's not there then off the cliff you go. Great compromise between the traditional tracks and the once-intimidating Rainbow Road.