When I bought HL2 I had never played any of the so-called "shooter" games. Strictly a sim-sort guy, racing, flying, military. Realism and more of it....
But after all the great reviews I gave it a try, and found it to be one of the most immersive and intense games I'd ever played. I know, some don't think much of the Valve/Steam interface, or being logged on in order to play, but I've never had a problem with any of that. If PC games are going to continue to be a part of the market, such measures are going to be the norm. (recent article on Slashdot about software firms trying to re-invigorate the genre)
Terrific visuals and graphics, great voice-acting, good story...What's not to love?
Also, being somewhat older than the average gamer, I found the "somewhat" slower pace to be more fun than the usual high-twitch-factor shooter.