Well for one, it was basically Oblivion with guns. Oblivion was a really flat game, it felt like "flatworld" if you know the reference. Lots of nice graphics but very little story. The level scaling was just the little extra that made it extra bad (yes I know there are mods to remove that).
Second, I did play Fallout 1, 2 and Tactics. Fallout 3 has nothing on those three games. I replayed Tactics not so long ago and it beats FO3 hands down.
One example of what made FO3 bad: you walk on a desolate field, nothing around you and then bam suddenly there's a huge monster 10 meters infront of you.... ruins the immersion totally for me.
In general Bethesda are good with sand box games, they dont do content/story. I for one dont like the pure sandbox games. I really tried FO3 but in the end it just felt like a 3rd grade game.
Bethesda butchered Fallout for me. I do hear that New Vegas did redeem the game a bit but I have yet to try it.