What the hell new enemies are you seeing in NSMB?

Huckit crabs, bramballs, bulbers, cooligans, ghost blocks and ghost vases, dragoneels, flipruses, and waddlewings are all new. There's also Nabbit, if you consider him an enemy.

I don't see the problem with variations of existing enemies. It's the mechanical application that matters. It would be a waste of time coming up with a brand new character for a bullet bill that behaves differently. Furthermore, I'd even argue that making a new enemy with a similar mechanical concept to that of an enemy that already exists a variation of the latter would be better design. When the player sees a boohemoth or some new type of bullet bill, they'll know what it's most basic behavior will be, and will be able to focus instead on reacting to the behaviors that are new or different from their cousins.

Additionally, variations of existing enemies also serve to develop the game world. Of course, I also love to see new enemies, but keep in mind that the NSMB games take place in the Mushroom Kingdom, so it makes sense that we'd see a lot of members from the same family.

The fact that, aside from going from grainy DS to HD Wii U, it's the same.

Fair enough, though that was probably done for the sake of marketing, and specifically towards casuals. It's probably easier to use a style that everyone is familiar with instead of making something radically new that might 'confuse' them.

Superficial it may be, but visuals directly affect enjoyment for people on an unconscious level.

Yeah, but if it's unconscious, how would you even know? I'm not denying that visuals can hamper one's enjoyment of a game, but I do believe that with the exception of crude graphics that actual affect play-ability or an aesthetic that turns you off for reasons 'outside' the game (e.g. extreme violence), you should be able to look past the aesthetics in order to appreciate the more important elements. Hey, I initially (stupidly) wrote off NSMBWii because of the visuals, but I was intrigued into giving it a try, and after actually playing it and witnessing just how exceptional the level design was, the visuals started to grow on me. I still think NSMBWii is rather bland (though technically impressive), but NSMBU is nice-looking.

Like if someone was sick of seeing Toon Link Zelda games.

I think that's mostly because most of the games with Toon Link are rather graphically ugly and don't do the art style justice (the 3D ones anyway, Minish Cap is lovely and FSA is decent). Even Tri Force Heroes being developed for the vastly superior hardware of the 3DS, just looks like a sharper, somewhat more vibrant PH/ST.