More storage, a better screen, smaller unit size and as we’ve discussed before better compatibility with games from non-Steam sources like GOG with a UI that unifies different sources and emulators (though I believe I read last month that Valve has alluded to them improving the non-Steam game functionality in the Steam client, so this could be a moot point by time the Deck releases).
I think if you’re entrenched in Steam’s ecosystem the Deck’s default state is going to be great. Yeah it could do with more storage, but in reality it’ll be fine with a bit of fridge cleaning.
And the OLED screen is almost guaranteed for a future version of the device.
But as soon as you have other use cases you’re going to start hitting more walls and potentially making your own compromises and sacrifices to use the Deck how you want to. That may include installing Windows over Linux, or as Valve warned against, opening up the device (and diminishing its drop resistance) to change the SSD.
But I guess that sort of thing can be a strength depending on how you look at it. Valve has often talked about a “turnkey” solution and this is the closest they’ve come to it so far.
I also think some of my criticisms are also things you can level at Steam writ large, not just the Deck.