Refresh features include:
OLED
HDR
90 hz
Better battery life (50wh vs 40wh)
Cooler
Lighter
Even better performance, even though not advertised by Valve! (because of the better DDR memory 6400mt/s vs 55000mt/s)
Bigger screen
Depends. How techy are you? None of those games are officially supported on the Steam Deck, but Valve lets you dig around under the hood and install whatever you want. However, anything that isn't on Steam and officially supported will require some work for you to get running. That said, you can play just about anything on the Deck if you're willing to put in the time. The touchpads make the Deck very capable on traditionally kb/m games.
For what it is worth, I’ve had a few games that weren’t officially supported so I just turned on the Proton setting and it’s worked so far. Online multiplayer seems to be a sticking point though due to anti-cheat.
It's really mostly EAC and Co nowadays that are blockers. And this is not because there is no support per se. It has to be allowed though because this stuff does indeed detect that it's not a "real Windows".
Stupid launchers are also trouble sometimes. Looking at you EA!
Anyway, a good source to tell is still the protondb. What's listed there usually works on the Steam Deck too. Or has workarounds explained.