Most of them are upgradable, even the old SNES had an expansion slot under it, but I never saw any use of it. Here in America, we rarey see upgrades for our game consoles because there isn't a great market for it. In japan many people would buy nearly anything so most of the weird upgrades are found there. Besside the N64's memory upgrade unit did anyone ever found an upgrade unit for any onther console?
As for friendlyness toward programmers, I heard the PS2 was a pain compared to the upcomming Game Cube (I prefered the old codename Dolphin). One thing we all learned from Nintendo (with the N64) is that no mrket is won in advance, it mostly depends on game manufacturers so they should watch out.