Now if only someone would do this with Commander Keen

Pretty much.

Including 2D.

Well, that's true. If there were more engines out there being used, it would be harder to distinguish them.

As for "hiding the look...." well, I'm certainly not an engine expert, but I think it depends. The ugly default Unity sunflare? Yeah. You can spot it instantly, but it's really easy to modify (and it SHOULD be modified, in my opinion). But something like the wind effects for terrain grass billboards? Not quite as easy to spot, but still something you can pick up on. And much harder to modify... if there's any real reason to modify it in the first place.

Similarly, I remember back before Unreal 4 and Unity 5, one of the most telling differences between Unity and Unreal III was the lighting. They just looked completely different, even at a higher level. I'm sure someone more savy than I will know why.

The other part of this is that there's seldom any reason to avoid every possible "tell" in an engine. For example, Unity 5 now has a new "procedural skybox." It's basic at best, but provides a nice foundation if you want dynamic weather or a day/night cycle. So why not use it?

You can also be on the lookout for things that would be caused by engine limitations. Weird flashlight shadows that cut off when you get too close to something? Unity game. Texture loading problems? Unreal III game.

Basically what I'm saying is that you can certainly change a lot with an engine like Unity, but I think there's generally going to be similarities between games made on the same engine. Because without a specific reason, there's no...well... REASON to avoid those things.
Yeah, you can certainly stretch it for all sorts of looks.

Placing poor made Mario model into some prerednred scenes in Unreal 4 engine is not the same as showing how good new Mario game could look on modern consoles. This is an outrage, bad taste and waste of time.

That day might come if Nintendo ever does a Super Mario Galaxy 3 for either Wii-U or their next console.

I don't get it... Mario Galaxy (and Galaxy 2) look better then this. I can't vouch for the Wii U title as I've never played it, but I don't quite see the point that they're trying to make...

And as for the Engine discussion, while I'm sure it's possible to make a platformer in Unreal, the real question is why would you want to?
I wouldn't count myself to be any kind of programming genius, but any attempt I've ever made to try and do anything that's not an FPS with something like Unreal or Torque has been a waste of time, why would you try and adapt an engine designed for FPS to make a platformer instead of just using a better suited Engine (or, for a big company with a lot of staff, make your own)?
I was quite impressed with Torque, until I followed the tutorials for making the basics of an RTS and a Tactical RPG (it was supposed to be like Valkyria Chronicles) and realised that trying to move a character not in first person didn't really work (presumably it would be fixable, but why bother fixing a problem when you could just avoid it by using a better suited tool?), which rules out most of the things I would have wanted to do with it...

damn unreal 4 is looking awesome, the character implementation was nicely done

I guess the reason this looks so tasty to me is because it uses Super Mario 64 sounds and moveset. It's nice to see that specific Mario in such a shiny polished rendition.