I expected the version to have at least as much content as melee and if things weren't on melee then they would have to make it up on something else.
So no adventure mode, we don't have it on both versions anyway but still. I'd say smash run is something they can make up for it since both of them are not long anyway and you get a different experience each match by enemies, stats, door placements, cannons at different spots, ect... on smash run. Not only that but beating a lot of enemies is fun especially with more stats you have.
Target test? Oh boy, replaced that with target blast? Blast is ok but it's not right to have target test not on stadium anymore. However, it makes a return on smash run. Not the best and incredibly short obviously, but I'm glad it made it in some way. So blast + smash run target test is a make up for me.
Trophies. This version already has more than melee and even brawl. Can't be disappointed by that and the trophy rush is probably the best trophy game in the smash franchise so far.
Special mode. Completely absent on this version but I believe the customs itself are good enough for me.
Classic. It still bothers me that they call it classic in this version. However, it is still fun and you do choose paths to whatever of the 3 you like and master core is just the best boss I faced in awhile on smash. I just don't like it being called classic. Wii U version though is much further from classic.
All star. The true all star is still unlockable from getting all characters and this time no items! Thank you! I can't express how many time items screwed me over. It may be easier but you only get 1 chance on this which is fair.
Event mode. Nothing can replace that one honestly. And no coin matches (don't even know if many people play it anyway).
Multi man. 15 minute smash is gone. It makes sense since this is on a portable. I don't see many people going for it on a handheld. Instead we have rival smash which in my opinion, is a great trade.
Characters. It's obviously this game has more than melee and even brawl. Not to mention mii fighters is a + too and is optional to have so those of you who don't like miis don't have to worry about it except them being the multi man team.
Stages. It has more stages than melee. Even if you cut out melee past stages I believe there are still more original/brawl stages than there is all the stages in melee.
Those are the things I can think of that I compared to it from melee. Reason I compared it to melee is because I know it's a lot of people's favorite. There are also things 3ds has that not even melee has to make up for it. Such as online, group modes, make your own (mii) fighters, challenges, replays, controller for smash wii u, street smash (I don't think anyone cares about street smash), and tips.
If I compare it to brawl, it will be a much harder subject to do and I will not try to discuss it. It may just depend more on your priority such as subspace on brawl for example.
But I can say it did better than I expected. I'll even go as far to say I like this version better than any smash game except the Wii U version.