From Melee/Brawl:
-The Offline Tourney Mode
Probably the best thing to ever happen for smash. Don't really know why it was cut in smash 4. Seriously tho, online tournaments were a disaster which took hours to start only to be killed by that one guy who spams corrin. (FML)
-Another adventure mode
Melee did it well, and Brawl made it fun and immersive. Why not try again? After all, there's almost no way they could screw it up with such a great track record.
-Certain characters
First of all Wolf, my brawl main who was cut just 'cause the devs don't like him. Snake, though his only chances are if konami allows it. Ivysaur, because I liked it in brawl. Plus, it isn't a clone, which is also good (considering the amount clones in smash 4). Finally, Pichu. Might be similar to Pikachu, but not to be taken lightly if utilized correctly.
-Some stages
I'm talking Fountain of Dreams, Kongo Jungle, Icicle Mountain, Spear Pillar, Brinstar Depths, Great Bay, & Pokefloats.
-Gameplay Aspects
Trophy Tussle Events, Actual requirement to work for CDs, Challenges more like Brawl, Wide collection of trophies, Trophy/sticker coin launcher thing
-Definitely agree that there is almost no unlockables in smash 4
Probably happened because nintendo keeps trying to make it easier for younger audiences. (Which is probably why it includes a few odd character choices, as well as nerfing the crap out of some of melee and brawl's top tier characters.)
Now for 64:
-Original Target Smash
Sure the target blast mode is ok, but it really leaves a gaping hole where there wasn't before. After all, combining target smash and homerun contest just doesn't work.
-Stages
Listed as follows: Mushroom Kingdom, Saffron City, Congo Jungle, Dream Land, & Hyrule Castle. Sure some were already in smash 4, but as paid DLC.
-Elements from the classic mode of each game.
By letting you choose a path/stage on your own, Smash 4's classic mode is just too easy and missing the things that made it "Classic" in the first place. First of all, replacing the waves of fighters with miis is just boring. (This applies to Multi-Man Smash as well). Also, there's not any fun challenges like catching the trophies from melee or smashing the targets from original smash games. Also missing is my personal favorite classic stage, race to the finish.
Things that should never exist:
-Tripping
Really annoying when you're running up to use an up smash and just randomly slip like you hit a banana peel
-Ledge stealing from SSB4
From a gameplay standpoint, this can cause a lot of BS moments in serious battles.
-DK's final smash
Seriously though, it makes no sense that his FS comes from a spin-off series of maybe 3 games, especially when this kong has had so many main series games.
kirby's FS in Smash 4:
This FS isn't nearly as fun as brawl, and makes him more of what would happen if you combine Jigglypuff with Ike.