Yeh - killer features are a big bonus. Thing is they are cool to work on - and so quite a few seem to work appear, pulling people in. With OSX and Windows that's fine because IMO they have that extra level of ease and Usability that a consumer OS needs. Linux doesn't have that left - I bet a lot of people drawn in with a gimmick are left struggling with it after a few weeks...
I still maintain that Linux is lacking a simple UI to pull it into the mainstream (ALL Linux not just Ubuntu).
But again... Quality of your UI software isn't everything. Sometimes a game-changing feature appears out of nowhere. Apple's multi-touch has changed how I use computer dramatically. And it has almost nothing to do with the software itself: it's a driver+hardware combination that I personally find impossible to live without. OSX allows me to do a lot with just one hand, so I can hold a cup of coffee in another: flip pages, zoom in/out, scroll up/down, etc. This was enough for me to stick to OSX for my daily non-programming computing needs. I even started reading books in PDF format, something I previously could believe I could do.