> You often can't decorate much.
No issue unless you fix it back up when you move out.
> Can't fit better windows, and windows where I've rented have usually been poor quality.
There must be laws in Germany, because I don't remember living in an apartment where the windows weren't top notch. That's also very necessary with temperatures sometimes going below -10°C.
> Can't replace the washing machine even if it's not working properly.
Like all appliances, furniture, anything, it's "bring your own" in Germany (unless you have a common laundry area... not super common I think).
> Can't put up shelves.
Same as before, as long as you fix up the holes when you move out, not an issue.
> Can't fit solar panels or better insulation,
Don't know about solar panels. Like windows, never lived in an apartment without proper insulation in Germany (in the US on the other hand...).
> or replace carpets.
You can put in whatever floor you like as far as I know, and I've done so.
> Sometimes you can't even paint the walls.
Definitely not an issue in Germany.
This confirms my impression that renting in the UK is indeed very different from renting in Germany. It seems like the UK also treats renting as more of a short term arrangement, while in Germany people may spend decades or their entire life in their rental apartment, so it's their space to create.