Having been in a conversation about Burning Wheel, a game built very tightly to a specific specification, I think back on how amazingly modular Rolemaster is.
Anyone who have seen the long series of companions published for the system probably know what I mean. Almost any part of the system can be tweaked, added to or subtracted from. It would still be Rolemaster! I think I have at least four different initiative systems in my collection, and that not counting the fan made stuff free on the internet!
I think that was the same thing that made me a fan of GURPS back in the day. Take the parts you like and the core is still the same. Learn some basics and then you know the game, but there's more flavour if you need it.
Contrast that with modern iterations of D&D.
Do you remember how convoluted, complex and contradictory the 3rd ed D&D rules became when you started hacking them with all the splat books that came out?
I still look favourably on Rolemaster, and even though I don't expect to play it again soon I almost always start make up new and exciting rules tweaks as soon as I start to read it. I like that quality in a game.