There are a few things, but ill mention my personal favorites from each gen.
quick resume: it's a game changer and playing on a console without it really shows.
BC: obviously not the first console to do it, but MS being dedicated to it after Sony and Nintendo abandoned it the previous gen was a wise decision. It was enough of big deal for Sony and Nintendo to go back to offering it.
Achievements, controller, and online: The 360 was the peak for Xbox and we got some great stuff out of it like the following.
Internal hard drive and Xbox Live: Internal harddrive gave the original Xbox a nice advantage over the competition. Xbox live paved the way for online gaming we see today.







