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.
I went for the biggest storage option and I'm not sure I'll ever fill it up, but mainly because I have no intention of using the deck as a catch all gaming device - it's my PC-handheld, I'll play games that are very ideal in that form factor. I'll leave Hitman and its' 150GB installed on my normal PC. For people that don't have a PC they game on, the "small" deck is probably a really bad investment, but the bigger ones should be as manageable or better than Switch or even phones.
Better screen I can agree with, it's definitely my only real criticism. Size I disagree with entirely, it's more or less perfect size for me (and the controls). To me it sounds like really, you want more of a Windows device anyway? Like sure it'd be nice if the deck was the one thing perfect for everyone but that's clearly not the goal, and in Valve fashion they are still allowing people to repurpose it anyway which is more than can be said for almost every other company in existence. Most of the non-Steam games I'd be interested in are in my itch or Twitch accounts and those aren't exactly high profile clients to make work somehow.
My single biggest fear for the deck doesn't have anything to do with the hardware - it is that they just don't support the software well enough - it's still under revision so updates keep coming but once it's out in the wild is the real test. Big Picture Mode was pretty much sent out to die years ago and still lacks fundamental functionality that you have to work around (like browsing the actual list of guides for a game, or it booting itself back up when you power off a controller when Steam is in focus, etc.)