Err... where do we start?! :-)
I guess given the ST's resources, it did its job OK, even though it was based on CP/M 68K.
The GEM Desktop GUI component, on the other hand, well... That really was nasty, especially if you didn't like green!!!
The ST's resources wasn't that different from that of the Amiga, except for sound and graphics. Resources of that kind - in this context (i.e. when it comes to OS complexity) - has very little to do with it.
I agree with Stopthegop - it was a minimalist approach. At the time, it was common practice to bypass the OS anyway (true for the Amiga as well, afaik).
Personally, I find that intense green colour just as annoying as dark blue..

btw, later incarnations of TOS wasn't even green.
-- Peter