GNOME? The great out the box experience... that also allows power users to deeply customize it? It is dishonest to lap GNOME (extremely popular free software) in with Apple (king of proprietary software).
The Steam Deck is uniquely positioned to allow people to do potentially hazardous things like install software from unknown sources, and that is because it's primarily a gaming device. People aren't doing banking and work stuff on it, so what is the damage if it gets rooted? Valve could have easily screwed it up by building a walled garden anyways, just to satisfy corporate greed in the name of "maximizing ROI". Sony, Microsoft and Nintendo all have their walled gardens for gaming. The fact Valve hasn't (yet) is refreshing.
You know its wrong to want to theme your distros desktop don't you? It is such a bother that you always ask your file manager to wear green. You know he doesn't like that can't you see how uncomfortable you're making him?
Prominent developers have variously spoken out against various user and distro customization suggesting that all theming and extensions be removed and even in one notable instance attempting to bully an app that supports many desktops to disable traditional tray icon functionality on other desktops using the dystopian phrasing something on the order of you should decide if you are a gnome app or a XFCE app. Another suggested that it would improve brand awareness if distros couldn't theme their version of gnome. Phrasing only a soulless marketing manager could possibly love.
There can be little doubt that IBM/Red Hat is about as hostile to diversity of options on the Linux Desktop as Apple and not without cause. There are legitimate arguable benefits to a homogeneous ecosystem. They just aren't in quite the same position to dictate options. For instance killing the extension/themeing ecosystem would have empowered forks and as few people would be running gnome as run Fedora.
And the tweak tool, sure but really the options in there should have been standard settings from the start. This is the core problem with this product.