I'd imagine the problems with the voice acting are all conscious decisions we just don't agree with:
- Cloud is Japanese only because his voice actor is unionized, and they didn't want to/couldn't go with anyone else (and also there were no loopholes like with the Kid Icarus: Uprising crew), Sephiroth is the same for consistency, and Sonic only has reused voice clips for the same reasons.
- Donkey Kong and Bowser were given their "voices" at the time and never updated either to reference the past or just because they didn't want to.
- Mega Man wasn't given a voice in order to fit with his NES style.
- Isabelle wasn't given a "voice" likely to be in line with the Villager.
- IIRC, Olimar didn't originally have a voice so he doesn't have one in Super Smash Bros., and it wasn't added when he did get one in Pikmin 3, and Alph is similarly silent to be in line with Olimar.
- Dr. Mario was a quick clone, so I imagine they just couldn't justify bringing back Charles Martinet so that his victory screens would sound better.
- Samus is mute to keep in line with her 2D appearances, and she's silent enough that I genuinely don't know if you can hear her while she's in her suit.
The main thing is that it looks super cheap, and I think the main culprit is the character animations. Like, when someone does an up kick it's like
BAM! the foot's up there and then
BAM! now it's not. There's no anticipation or followthrough. It's kind of a shame because the character animations are likely the most important part, and everything else looks quite good. Well...Stimpy's model looks a little rough...
The lack of voices doesn't help, but from what I've heard this will eventually be rectified, so that's a plus. Also, the soundtrack isn't all that great, but I'm sure most people are going to overlook that.