The good
PC gaming made easier for the masses
Competition in the store fronts - MS/Xbox, Steam, GOG, EGS etc and better digital prices
Forward and backwards compatibility, no need to wait for remasters/remakes
Wider and better support for peripherals and display technologies
Mods
Free online play
Full function of Discord and other apps
Its a PC - You can work, play
The bad
Optimizations won't be as good as strictly closed hardware
Price. This is not going to be cheap. Expect $800-1000
The unknown
Longevity or long term support for hardware
Shader optimizations. Hopefully the new DX will have something in place like Vulkan as there isn't really a need for Shader compilation, even on modest hardware like the Van Gough APU in the Steam Deck.
I expect a more optimized and streamlined PC experience just like Valve is doing with SteamOS and the Steam Deck. An OS and software centered around gaming as a focal point unlike the hodgepodge of quirks that come with current Windows. I think that would make for a great companion device for a lot of enthusiasts like myself that want a powerful portable PC, with less downsides. Or a great alternative device for new buyers that don't want to deal with current PC stuff and want a high powered console for gaming and/or already have a NS/2 or PS.