Balance issues can be overcome, but the real issue I see with it for the hardcore crowd is as follows:
1) The mechanics, while sound, are too simplistic. Every character has a fairly limited amount of moves without a great deal of nuance between them. This kind of limits diversity and people will just gravitate towards whatever character has superior priority in most situations. This will lead to the same bullshit of everyone playing Kung Lao that MK9 degenerated into in its first year. MvC has some of this issue, as well, but you need a math degree to know even half of the differences in SF4 characters (or any SF game, really) which makes the experts at specific characters much more interesting.
2) The executions are generally too easy on some of the better characters. This really means that there is not a lot of gap between the hardcore elite and strong casuals, once people learn all the matchups. This means that on the first issue, the hardcore ADD Asians will not be able to separate themselves from the masses as easily by taking a more nuanced character, which is what you see in SF4. MvC sort of has the opposite issue where every match is a setup to a 100% damage combo and the guy who wins is the one who does not fuck up the ridiculously hard execution.
I do not think this game is going to have staying power with the hardcore crowd, especially the Asians. In fact, MK9 was probably a better hardcore fighting game, once they toned down Kung Wow and Raiden a bunch. As a casual game, it is probably the best casual fighter to come out in ages. Its fun to play and visually fun to watch. For me, as a casual, MvC bores me to tears as its the same half dozen characters doing 100% rape combos all the time and the casual play is so massively different from the expert play that there is no way for a casual to relate to the high level play. SF4 is more accessible, but has some really nuanced executions and a ludicrous amount of matchup specific knowledge if you want to ever progress past the "spam super move" stage of play, though it is still probably the best game in the genre. Really the issue with the Capcom games is that you have to play them as a way of life to enjoy them at all, while the NR titles more or less can be gradually picked up and much of their depth experienced by casuals. The NR games are not the darlings of Evo, but that's more or less a giant japanophile stroke fest packed with frat boy mentality, anyhow, so the lack of tournament exposure does not reduce my enjoyment of their games.
I really like Injustice and am enjoying playing it casually, more than I ever did SF4 which I still play occasionally. I think I liked MK9 more, but I need to spend more time and see the first couple balance patches to make a final judgement. I enjoyed the Capcom games, but lets face it; those games are complex to the point of pretty much being pro level only to get the most out of them. Injustice has the potential to expand the fighting game market in the same way WoW opened up the MMORPG market, by being an easier more casual friendly product with solid outside intellectual property to drive interest. My biggest gripe is the lack of 3d graphics (something I enjoyed immensely about MK9) and the balance issues. As long as they fix the balance at a faster pace than they did with MK9, I will continue to play it.