Maybe you should play the game yourself. I also thought DF was full of it, but the fact is I also notice it. When more people get their hands on the game, I guess I'm not the only one. I dunno what it is...seems like a frame is skipped or a kind of stutter.
After playing the game, I really must say the DF did a great job to describe this:
"During gameplay we experienced the regular appearance of duplicate frames manifesting as
a constant but subtle stuttering effect. Upon analysis we determined that the game suffers from extended clusters in which a duplicate frame is displayed every 64 frames. What this ultimately means is that, during normal gameplay, Mario Kart 8 continually drops down to 59fps.
This may not seem like a big deal - most will probably not notice it at all, and it has zero affect on playability - but it has a noticeable impact on image fluidity that mars what would otherwise be a perfectly consistent frame-rate. And for us at least, once it is seen, it can't really be unseen."
Edit: beaten to it. Seems that I'm no mutant after all