Take Oblivion, give it some guns.. and oh yeah... its set in a landfill. Its not a pretty game by any means.
Having played the first two Fallout games, I felt it would be a crime to skip over the first one that managed to make a splash on the gaming scene. Fallout 3 is true to its roots in virtually every way, in that the setting is the same, characters are reminiscent of the past games, and the overall quest is very much akin to Fallout 1 & 2, but instead of being a point and click adventure (think Diablo), this game is a first person shooter/adventure very similar to Oblivion.
This game has all the things that made Oblivion great, except for the beautiful atmosphere. Basically, the biggest difference between these two games is the setting, and I don't think there are many people who can disagree here: Oblivion takes place in a beautiful place with sprawling forests and lakes, expansive castles, wildlife, and basically everything that makes a hike through one of Americas national parks a beautiful experience. Fallout 3 takes place in a landfill. In this, I think that Fallout 3 is a step down graphically. Good graphics should involve more then resolution, polygons and pixels.... a game should be pretty to look at. Fallout 3 is an eyesore.
Another complaint about Fallout 3 is that it is brutally hard from the start. Take Oblivion, turn the difficulty all the way up, and you have an experience akin to Fallout 3.
I feel that I've made a mistake spending so much time complaining, as this is a truly great game. Don't look at it as "ugly, and hard", so much as "An unattractive and difficult variation on the greatest game ever made". Buy Oblivion, and if you have it already, Fallout 3 should be on top of your 'To Buy" list.