The expansive world is probably the best feature that comes with Fallout 3. The map will probably require about twenty minutes or more of walking to go end to end and you'll usually run into many abandoned buildings and towns that have a lot to explore. A big part of the game is actually the aspect of having to scavenge the ammunition and first aid packs that you'll need. This actually makes the beginning of the game a really unique experience, where you'll need to make every bullet count. One of the worst aspects for me was the slow disappearance of the “wasteland raider” feel that the early parts had.
The VATS system is another big feature. It's supposed to be a mixture of the old turned-based combat and the new real-time combat. In the end, it's basically just a cool feature that makes for some cinematic shots with slightly better accuracy. It often breaks the flow of gameplay and makes fights too easy. The system basically just has the game go into slow motion and carry out a few attacks for you. That said, it is always fun to watch a rifle round travel in slow motion until it blows your opponent's head apart.
There is a skill system in place along with perks, but they weren't that satisfying to be honest. The problem with the game comes in the form of a strange break of reality. My “small guns” skill shouldn't matter if I shoot an unarmored raider in the head. In the end, it just works to push up some arbitrary numbers and just doesn't feel that necessary. It doesn't even have much of an impact. Besides lockpicking and hacking, none of the skills really felt like they made my character better. It just pushed up the effects of a few items.
The quests are also a bit of a one trick pony. There are a few cool options, but they usually feel like the same fetch quests that Oblivion used ad nausea. They are alright as a method of driving the plot, but it just didn't feel like there was much of a consequence for the any of the choices. The worst that usually happens is that the game's radio station will insult you over the radio. It doesn't feel as good as it could have been.