With regards to density, San Jose has a density of 5,256 people per square mile. In contrast, Denver (which has an awesome train/bus system) has a density of 3,698 people per square mile.
If low density is why San Jose can't support a decent transit infrastructure, then how can Denver do it with even less density?
Just my guess, as a Denver-area resident... Metro Denver has several areas with very high concentrations of office space and jobs (Downtown, DTC, Broomfield/Interlocken, Boulder, etc.). Also, the residential density (even within Denver proper) varies a great deal. The lower-density areas probably don't have great bus service. (The residential density also seems to be increasing, with a lot of infill development projects.) It helps that the Denver area is known for having strong regional cooperation with a single transit authority, instead of the crazy mix of poorly connected services in Silicon Valley.
Silicon Valley office space seems to be a little more spread out. When I was visiting a client in Santa Clara last year, I had to walk almost two miles to get to a light rail station to go have a nice dinner in downtown Mountain View. (And there were no buses available to cover that distance -- at least not that Google Transit knew about.)
(Disclaimer: My experience with San Jose is mostly limited to the airport area, and the Silicon Valley suburbs to the north.)
I live in Denver and second that the light rail / bus system is pretty awesome. It should be even better with light rail expansions coming in the next year or so.
I live in Denver too and I haven't driven to work in 6 years. Instead of driving myself, I read, listen to music and chill out. I also don't worry about traffic, weather, or really much at all. It's great for drinking after work too.
I share your enthusiasm for the upcoming expansions, in particular the line to the Airport. The train is one of the big perks of living in Denver. It's awesome and only getting better.
If low density is why San Jose can't support a decent transit infrastructure, then how can Denver do it with even less density?