Not all that much of the OS was assembler - the biggest piece was FFS (which subsumed OFS), and honestly probably shouldn't have been in ASM - but space was *tight*. Sure, quite a few of the drivers were in assembler, and performance-critical parts of Exec were in ASM, but that was almost required at the time for low-level HW interfacing. Much of the OS was in C. (I was responsible for removing the majority of the BCPL code (look it up on Wikipedia) used in AmigaDOS for OS 2.0.)
It was all fairly carefully designed, and a lot of work went into making it bulletproof and snappy. While there are huge benefits to memory protection nowadays, most Amiga programs and certainly the OS were quite resilient to pressures, such as allocation failures, which would crush almost all apps today. Error paths were much more likely to get tested, and the path wasn't the library calling exit(1) for you when an allocation failed.
That said: it's 15 years behind the times now. No major improvements have been made (some, yes, but nothing major). Dave is basically right - and we were in the last year trying to break with the old hardware design, though there was one last big step left in it that actually got to the early prototype stage (AAA). We hadn't planned out where software would go, but if you look at what Apple did you probably get a hint of what we might have done. It would have been tough, though, since we didn't have the resources to throw at emulation at the time that Apple did. In the last year, the SW group (which I ended up running a good part of) was down to a handful of people ( 10 I think). I think the "OS" group was down to maybe 3 or so. The writing was mostly on the wall by around a year before *poof*, and much of the team left in '92-93 to places like Scala (where many still are, and where I went after bankruptcy), 3DO (which had a strong ex-Amiga and ex-Commodore influence from the start), etc.
I wish it had been open-sourced back in '95 or so. It may not have survived intact, but it might have formed the core for a strong competitor to Linux/etc and at least pushed them to improve their responsiveness much earlier on.