On the subject of previous Mario Kart's, I'd like to note that the general slide/speed boost combo is more in line with the Wii release. The DS version favoured a technique known online as "snaking", which could effectively give gamers an indefinite number of minor speed boosts throughout each track. This time around, minor speed boosts come from more natural drifting, and are far harder to replicate down straight areas of the track. I should also note that in previous games, the level of speed boost was determinable by the colour sparks underneath the player's kart, but in this release, only one minor speed boost was achievable, which should help to balance things out in the long run.

This might be based on the E3 build, and is slightly incorrect. I've played a newer build and there are two levels of boost, but the rest is correct; it's more difficult to achieve--especially the more powerful one--and seems to have a less pronounced boost effect than previous games. So long, snakers!

Source: WiiLoveIt