From what I saw in the demo, every character had prospects other than Zelda who was just awful though the game is incomplete so I suppose it's too early to give up on even her.
The thing with a tier list is that it's the person or organization posting its subjective view on the value of the units being tiered based on their current knowledge. So when I posted a tier list for smash 4 based on my demo experience, I was posting how strong the characters seemed to me relative to each other based on what I knew. Honestly, as long as the characters remain different at all, I could complete this exercise even in a very balanced game; it's not a sign of the balance of the game to be able to estimate tiers (though seeing differences in different tier lists should be a good sign of incredibly tight balance).
No one can really tell you the balance curve of smash 4 now, but at face value, it seemed better than it had been before. Every character other than Zelda had a variety of strong things about them, and even the few who seemed more clearly lower (Mega Man and Rosalina) were complex enough that it was very plausible to assume that there was simply more to learn than could be learned in the time the demo allowed and that further these characters were obviously terrible in timed FFA and probably benefitted a lot from a more controlled 1v1 environment. Now, as games evolve, the balance really only gets "worse"; balance really never changes, but as we learn, we learn to exploit balance differences more which makes lower tiers relatively less viable with time. However, with both Melee and Brawl, a pretty broad class of characters were just obviously terrible and could be written off on day one so your starting point wasn't even that great. In this game, we get to write Zelda off day one (unless she is super-buffed from the demo!) but have to consider what so far seems to be the entire rest of the cast. In other words, there are no promises for the future, but the starting point seems optimistic.
In terms of the practical, what kind of character diversity you see in tournaments, realize that it's just as much cultural as a reflection on the balance. Here in the Midwest, people pick all kinds of crazy low tiers even to this day; the last Brawl tournament I was at had a Lucas player, a Wolf player, and a Samus player, and MK was only something like 1/6 of the field even including everyone who ever used MK as a MK player (that is, multi-character users who included MK in the rotation). To be clear, this is way more diversity than the game's balance really supports; it's just a sign that people are willing to put themselves at big disadvantages in the name of their favorite characters around here. On the other hand, if you go to Atlantic North and see a tournament, it's just plain different and people tier up a lot more, arguably even more than the game's balance really suggests they "should". So really, just as much as hoping for good balance, you also have to consider the cultures, and while you can probably nudge those, they aren't the most flexible things in the world. It's probably healthiest to just be happy using whatever character you personally want to use and not to worry about what others do...