Swimming and gliding really added nothing of competitive value. Swimming was just a gimmick that wasted more time, and gliding just offered an obnoxious polarizing/stalling recovery for the characters that had it.
Transformation changed up matchups, but most transformation characters were severely limited by several specific rules or hindrances. Pokemon Tranier's Pokemon were limited by the stamina mechanic, and the redundant elemental mechanic that seemed hastily slapped on. Samus could only transform into ZSS by either selecting her at the start of the match, using a Final Smash (not allowed competitively), or doing an incredibly precise button combination. And once you transform into ZSS, you can't chance back (barring a Smash Ball which is banned anyways). Ans lastly, even Sheik/ZSS was hindered by one large problem: Zelda sucks. So not only was transformation limited by several factors, intentional or not, but it also has three other problems. Balancing transformation characters, removing a down B JUST to have a transformation, and the fact that transforming in a match causes lag spikes. And additionally, how many matchups require legitimate use of transformations where you even want to use both characters to begin with? Most of the time you are still only going to stick to one character anyways. With all of the problems that transformations have caused, while also yielding little to no benefits, it is just far easier to have transformation characters as separate characters altogether.
Now then, if you want to talk about important mechanics that were removed, we have the following examples:
DACUSing, Glide Tossing, Aerial Glide Tossing. (Dash dancing is still possible but much harder to do now as well)
These things offered way much more potential than swimming, gliding, and transformations. Less so in regards to DACUS, but the other two were extremely important to learn and master. Especially for a character like Diddy Kong. They offered new ways to approach an opponent, to play defensively, added mindgame potential, and EVEN aided recovery thanks to AGT. THOSE are gone now, and I am more saddened by those losses than I am for swimming/gliding/transformations.
But on the bright side of things, we still have these mechanics:
Crawling
Wall Jump/Cling
Shielding during a dash
Up Smash during a dash
Up Smash out of shield
Jump out of shield (Which YOSHI can do now)
B Reversing
Reverse Aerial Rushing (RARing)
Footstooling
Hovering (Peach)
Canceling an air dodge with a Zair (Link/TLink/Samus)
Double Jump canceling (?)
And more
Plus we have these NEW mechanics to experiment with:
Improved dash canceling (?)
Pivot Canceling (includes Pivot Grabs)
New ledge mechanics
Overall, I feel as though that the good vastly outweighs the bad for this game, at least in comparison to Brawl anyways. Plus, tripping is gone. That is a mechanic change that I am sure 90% of players are super happy about, myself included. Everything about that mechanic was a toxic mess of nonsense.