"Pre-order DLC" is like everything else - done right, it's fine. Done wrong, it's atrocious. Pre-order DLC outfits are really quite fine - it's a little visual in-game trophy of sorts to show that you pre-ordered the game. Smash Bros Wii U could do exactly the same thing with a literal trophy (that wouldn't count towards completeness of your collection) and I wouldn't be bothered by it.

On the other hand, when there is actual, real content only available to those who pre-order, that's it done wrong. When it's a gift that changes gameplay, that's it done wrong.

So my attitude right now is "eh, I don't have a problem with this"... but if they go too far at some point in the future, I'll be pissed.

Yeah it sucks.

I disagree: I think it's always on the atrocious side. I see no reason why the actual content offered should change the analysis: trivial DLC simply makes it less offensive.

Why exactly is it offensive at all in this case?

Nintendo getting with the times. Next up: Season passes and achievements (I'm gonna guess that they'll be called "Badges").