1) No memory limits (technically, there is a limit, but it is much bigger than, say, app heap limit on Android)
2) Background tabs can use setTimeout() to indefinitely run code long after user left them
3) Websites can start background processes (Service Workers) without user oversight. Users can't prohibit that or kill them via easily available means.
4) Applications are non-optimized by default, need massive CPU resources for JIT-compilation
5) Rendering a page with five sentences requires executing tons of said unoptimized code.
6) No permission system for majority of things. All important user controls have to come with browser add-ons.
7) Applications can update themselves anytime without user consent
8) Most web apps do not work offline
9) Everything is tied to proprietary web services with overarching surveillance and dodgy EULAs...