They actually don't next to Xbox! Series S has all the OS features of Xbox Series X, with <2GB of RAM. Which could mean a richness in features similar to that. And that is a lot!
For me that's the exciting part - that much RAM is enough to allow media apps to run without closing games, it's enough for Quick Resume, and high quality captures, and streaming.
The successor will have 1+GB of memory just for the system, which Nintendo made some pretty fantastic use of on Wii U. In supported regions, you could even stream, watch live TV and more, along with a fairly fully featured web browser.
I don't expect a browser from them ever again, but I think there is a possibility of opening up HOME Screen Apps (Applets) to developers, rather than just NSO, eShop and News. Discord, YouTube, Netflix, who knows. Between the existing ones, we know Switch CAN play HD media in the foreground with a game suspended, but Nintendo never allowed a third party in on that.
That's a big thing in my view, this OS is very much the same OS. Expanded, more features, maybe, but they aren't starting from scratch. I hope it inherits the "good stuff", and that includes being lean. But as many features as they can squeeze in without slowing it down would be nice.