So I've put a bit of time into this over the last couple of days, not enough really to do more than scratch the surface but here are some initial impressions.
It really is at heart an updated version of the Wii U game. It plays the same and largely looks the same. It's maybe slightly quicker and some of the characters have been rebalanced but at heart it feels very familiar. That's not a bad thing, I think a game of this size has to be iterative.
This time round you start with 8 characters and have to unlock the others. You can do this in any mode - ten minutes of play on anything (e.g. versus mode with friends) and you get a "New Challenger Approaching" screen and get to fight the new character, if you beat them then they get added to the roster. I don't know if the order is predetermined or random but they're not unlocked in chronological order - we got Inkling quite early on and have been unlocking new and old characters as we go.
You can also unlock them in the Adventure mode which is - surprisingly - a pretty decent single-player mode. You travel around a map fighting a variety of themed battles to unlock and level up "Spirits" which can be attached to your character to give power boosts, special abilities and so on. There's quite a lot to it, the map is huge, there's a skill tree, a few basic puzzles to solve and some quite creative match-ups. It's still Smash at heart but it gives structure and a sense of progression to things and I can see myself playing this quite a bit.
There are loads of stages and a surprising number of new items and new assist trophies so you're constantly dealing with new stuff even if you haven't unlocked many characters yet.
I'm surprised at some of the stuff that hasn't made it across though. There's no Home Run challenge, no Break The Targets, the Smash Tour and Smash Run modes have gone, no Trophies to collect and most surprisingly you can no longer unlock custom moves for the characters (which was a pretty big part of the Smash 4 end game). You can play with the Spirits you've unlocked if you want - though it's not compulsory - so I guess that replaces the custom moves but it feels like a simplification. Maybe people didn't use the custom moves much; and I can certainly imagine it makes the game easier to balance.
Online has been lag-free for me so far (I'm using a wired connection though) but you can't choose particular match-ups, you can simply express preferences and then it puts you in whatever it can find. I've been trying to get 1-on-1 matches with no items but I keep getting dumped into 4-player free-for-alls. I think you can eventually unlock an Elite League which is a variety of the "For Honor" settings from the previous game so I guess I'll get there in the end.
The menus are absolutely baffling, as always. I thought All-Star Mode had also gone then I found it about three levels down in the menu hidden under something else entirely.
In short: it's Smash, there aren't many surprises here (apart from the fact the single-player mode is actually quite fun for once) but it's absolutely massive and there's probably weeks of play just unlocking the full roster. I love it, but then I always knew I would.