So I just got this this morning with my shiny new XL.
I've only just played the first world, but I can already say that I actually think the coin mechanic is a nice wrinkle, IF, you choose to "play the game" and go for high score and the million coins.
What I mean by this is, in the past few NSMB games, coins have been completely useless. To the point of I could be going through a level, see a big cloud of coins in the air, and not even be motivated enough to do the work getting to them because, who cares?
I've never really made it a point to get coins, but now, when that's a more key mechanic, I find myself playing the game much differently, going for every coin I can, looking high and low for little nooks where they may be hidden, etc.
It's not a major game changer, and so far the general game looks like a very normal Mario game, but I think the incentive to "go for the gold" creates just enough of a change in how I play to keep it fresh.
Plus it looks so nice on the big ass screen!