The idea behind the buffer is that it wouldn't take a button, it would just work that way automatically. You'd still have to perform the wavedash motion, the timing of the shield button just wouldn't be tight anymore.
Like I said before, controlling your character should be intuitive and natural. Knowing what to do in each situation is the part that should take practice and skill.
I can tell you're not particularly good at Mario Kart. It was snaking that allowed the casual players to have a chance, because it rendered more nebulous concepts like item strategy and race positioning less important. To casual players who don't understand these concepts, the outcome seems random, but if you have a high level player in a room full of moderate level players they'll win almost every time, hence the game is actually not very random at all.
If you want to learn a bit about how the strategy in high level Mario Kart works, you can check out the video below. It's got some pretty good commentary that explains some of the strategies that are in place.
http://www.youtube.com/watch?v=WReLnCEnow4
Except it's not really. Most of the most popular games are those where execution barriers are low. There's a reason Mario Kart is much more popular than, say, F-Zero or Wipeout. Similarly, there's a reason Street Fighter 4 was much more popular than Street Fighter 3.
I suppose you could consider something like Starcraft a subversion, but the entire RTS genre is pretty niche because it's so hard for new players to get into.
It's also worth noting that execution and competitiveness aren't necessarily related. A pretty strong case can be made that Mario Kart is actually more competitive than F-Zero. The mainstream fighting crowd also used to believe (some still do) that Smash is not competitive because it is not execution-heavy, but anyone who plays Smash knows that is not the case.