I really do believe that this game will have more than enough FPS content to leave you satisfied. 20 hour storyline, not including side missions. Fallout 3's storyline wasn't much longer than this, if that's all you did. Plus, once you've completed the story, you'll be able to go back and complete all side missions and pad stats. You may be able to put in 30 plus hours into this game without the extra content.
I admit, I was a bit surprised at the direction ID Software chose to take for multiplayer. But not disappointed. I think it'll be a great addition if they do it right. They still will have FPS multiplayer, but just not the traditional FPS deathmatch. They will have a deathmatch styled vehicle combat. I'm fine with it. Fallout 3 is probably in my top 10 favorite games, and all they had was the story line and some DLC. This game will have a long story mode, maybe not quite as long as Fallout, but they'll also have plenty of additional content. All I care about, is that the multiplayer is fun.