What Superior means is that a weapon is just plain mechanically better than other, similar, non-Superior weapons.
It's wrong to give away mechanical advantages to one race for no good reason.
But you're not giving a mechanical advantage to one race for free. The race has to use a feat to get that advantage. The advantage itself isn't particularly exotic- 99% of the time another race could duplicate it with two other feats, although the results vary according to tier.
(1) At heroic, the feat is better than two other feats combined. Definite Dwarven advantage.
(2) At paragon the feat is as good as two other feats combined.
(3) At Epic, the feat is inferior to two other feats combined.
Once the character hits 11th level, a Dwarf with this feat essentially gains Superior weapon proficiency for free- in other words, he gets a bonus feat. This is not exactly an overpowering advantage.
The variety available to the dwarf is a nigh-useless advantage. Really, is there ever a realistic point where a dwarf that can use two types of superior axes has an advantage over a human with the ability to use one type of superior axe? No adventurer is going to be investing in two +3 different weapons to carry around. The dwarven ranger can use two types at once, but you won't any sort of advantage in the numbers over a human ranger that is using two dwarven war axes. It's simply not a big deal- it makes for good flavor, is all.
A dwarven fighter or ranger simply doesn't have an overwhelming advantage over a human ranger- he's definitely better at wielding axes in the heroic tier, but he's no better at paragon and above. At that point he just has more flexibility in feat selection. Well, actually, he doesn't, since humans get an extra feat. But whatever- you know what I mean.