This is really a question of audience (or client).
If I was creating a site for unix users I would look at different set features to focus on (i.e. no frames for lynx, no plugins like flash,...) than if I was creating a site for an advertising firm.
If you want a broad base, but still be able to do cool things like flash, streaming media, frames and (insert current industry buzzword here) you can always use HTTP::BrowserDetect. The capabilities (or ability to use load plugins) of browsers is easily researched.
Unix - where you can throw the manual on the keyboard and get a command|