After playing a lot of Smash 4 I can say these are the things I miss from Brawl:
-a story mode in SSE; Smash Run is almost a good replacement but Smash Tour is just not worth the slot it occupies on the main menu
-several top tiers from Brawl not being nearly as good (Meta Knight deserved what he got even though I loved him, but Falco and Marth didn't deserve how hard they got hit, same for other characters who got nerfed for little reason and weren't even top tier like Game & Watch)
-more set pieces from the Stage Builder being available; Brawl had a lot to work with, SSB4 has very little by comparison
-Classic mode is kinda meh in Smash WiiU (it's a bit better in Smash 3DS IMO where you can pick to just fight Master Hand instead of being forced into whatever combo of Master/Crazy you get based on difficulty
-a couple specific stages, but for the most part most of the best ones were brought back either on 3DS or WiiU
For these reasons I may go back to check out Brawl's stuff at some point, but probably only to have a retrospective of how every Smash compares to one another. In every important aspect, Smash 4 trounces Brawl.