-No mods.
-No config tweaking.
-No reshade or any other graphical changes.
-No way to bypass bugs, you'll be forced to wait until developers fix them. If they ever do...
-No communities keeping games alive years after release by creating content for them.
-No using cheat engine to counter ugly balances in some single player games.
-No way to check/analize how games are made.
-No way to find secrets or things left on the files.
-No game conservation as we don't know if every game will be always available.
-No enough speed in most countries for this kind of streaming to work.
-No control whatsoever over any part of the games.
For example Skyrim wouldn't have survived so many years after release if Stadia was out back then, as no mods would've been released for it.
Google controls it all for you. Sorry but this is a massive no for me.
Pretty much this, I guess. From a consumer perspective, my reaction was a resounding meh - if this is the future, the future can suck it.
But it is new and shiny, so I'll certainly check it out.