Let's get something out of the way first; as far as pure speed goes, Smash 4 and Melee are practically on the same boat. Fall speed and dash speeds are quite similar between the two, and the area with the most noticeable discrepancy at this point would be the mid-air speed of certain characters.
With that out of the way, what Smash 4 needs are mechanics less favouring to the defensive player. Heavy emphasis on rolling can decrease the excitement of the game. When you have no better alternative than rolling to act as a safe positioning tool, the invincibility it just so happens to also provide slows fights down quite nicely. Wavedashing is an example of a positioning tool that's seen as viable alternative to rolling in part because of the speed of execution and follow-ups. However, should the wavedashing player make a mistake (such as a bad read), it was very possible for the opponent to capitalize and initiate their own attack. Rolling not only takes more time before it is effective to counter attack, but it also takes more time for the rolling player to initiate their own attack as well. The whole match suddenly turns into a roll-fest because by the time one player is able to attack, the other one has entered their own roll and become invincible, forcing the first player to roll again to approach safely (and the cycle continues for a bit).
A more obvious one would be approach options. Grabbing out of shield is too viable the way the meta has currently developed, and the lack of any workarounds to getting grabbed spells out problems for those who are too aggressive in their approach. What this means is that Smash 4 is a game that emphasises punishing more (especially with grabs when one considers just how many follow-ups Smash 4 grabs provide); players wind up doing everything but approaching to get the opponent to make a mistake. This could mean a lot of the previously mentioned roll-fest. Melee had L-Cancelling and wavedashing among other mechanics to avoid getting grabbed as easily and providing methods of fast approach. That is not to say that these mechanics need to return in Smash 4; the mechanics introduced in Brawl have its unique characteristics to make it stand out from Melee. I'd love to see a competitive smash title (that's also fun to watch) that's not just Melee or Melee 2.0. Brawl's (and by extension, Smash 4) mechanics are far from impossible to work with; there are ways we can speed the game's fights up through reworking things like ending lag or sliding in shield.
