I haven't even touched the Main Quest yet. I started on the Survival Guide quest out of Megaton, and have acquired many of the other side quests in my travels for the Survival Guide sub-quests.
I am greatly enjoying just wandering around the wasteland, exploring various areas and taking on Raiders and Super Mutants.
By just wandering around, I stumbled upon a firefight between the Brotherhood of Steel and some Super Mutants. I ran into a pacifist Super Mutant wandering the wasteland named Uncle something or other. My favorite random encounter thus far was the Antagonizer quest...I kept her helmet, and still wear it simply because it looks so absurd.
Cleared the Mutalisk den underneath one of the monuments. Took on the Fire Ant quest in I forget which town.
Currently doing the Blade Runner-esque android quest out of Rivet City.
I am also enjoying how the radio station news is starting to reflect and report on my actions...a nice touch for making the gaming world feel alive.
I loved Morrowind just because the different environments were so unique. Oblivion was a better game graphically, but the world was a bit dull and too medieval realistic as opposed to high fantasy. Fallout so far has been great.