Whoever came up with the coin and super horn items needs a raise or a promotion - they'd seem lame on paper but they're brilliant. The piranha plant and boomerang are also great. Each of them serves such an obvious purpose in the game design that makes the entire game so much better.
Coin: Reduce the leader advantage inherent to MK (it's still an issue, but it's reduced). This has allowed them to turn the amount of blue shells WAY down, while keeping the leader close and increasing the value of the red shell hugely. It also makes leading a race far more fun, as you can't go on autopilot any more. The coins are also more apparent in how much faster you go this time around. You really want to have 10 coins. It's worth it to go off the racing line every now and again to get them.
Super horn: Again, this is a 'leader's item'. It's to entice the leader to drop his defense in the hopes of getting this to defend a blue shell, but if he does so he may end up with only a coin, meaning he has no defence at all. There's risk and reward there. When you risk it and do get the horn, knowing you'll be able to see yourself home in first, that's a hell of a feeling.
Boomerang: An item that can break through defense. This was needed.
Piranha Plant: A much funner way to speed up a bad player and attack others than the Bullet Bill, which is now as rare as gold dust, and thankfully so.
I know it's no surprise but why do they make the 150cc AI so bloody transparent! 90% of races end with a blue shell and/or a barrage of similar shit on the last few or even very last corners. After coasting for 2.5 perfect laps only to have ur 3 star rank get shot to shit... It's just not worth it. Having the ranking is not an indicator of skill it's a sign that the player is a goddamn masochist!
This is the exact opposite of my experience. Blue shells are rare as hell in single player, and in 150cc you can be half a lap ahead of the computer so it doesn't matter if you do get hit. 3-starring these grands prix has been really, really easy imo.