It would be an insult to just call it 'Oblivion with guns'
+ Large amount of quests an exploration to do
+ Expansive stories and huge variety of armour and weapons
+ Many choices to make keeps the game open and fun
+ Challenging but still fun
Negatives
- The wasteland can become boring sometimes
Fallout 3 is made by the same people who made Oblivion, to the casual gamer it may just seem like Oblivion, but with guns.. to say this would be an insult!
It improves on The Elder Scrolls: Oblivion in a lot of ways, the quests are a lot more open, your given several options as to how you will over come the quest.. for example, in a small town you come across a group of people trying to defend against super mutants, then depending on your skills your given options.. do you help them with your science skills and build super robots to help in the defense? or do you use your skills in heavy weaponry to train them how to use weapons well?
The variety and choices and massive.. there are lots of weapons and armour variations, these range from leftover melee weapons like pool cues and sledge hammers, classic weapons like the SMGs, pistols, assault rifles, hunting rifles.. and you can even pick up more advanced weaponry such as laser rifles and alien weapons, most are optional making you go out of your way to do optional quests..
Often you wont be strong enough to progress in the main story which forces you to do more side quests, which usually go along the lines of shooting.. then making morale decisions and then running away onto your next mission.
The huge open world also contributes to this freedom, it actually feels like a post apocolyptic world.. you can visit supermarkets over run by bandits and see what's happened to monuments and museums in the American wasteland- in one mission your goal is to install a radio signal at the top of Washington monument- there's lots of fun to be had, altough the majority of the game is spent chasing your Dad after he abandoned you in the vault forcing you to escape.
As you kill more things and discover more locations you gain experience.. which leads to level ups which allow you to upgrade your stats, these stats include sneak, science, big guns, small guns, energy weapons, melee weapons and many more.. each lead you to become a unique character though.. what sort of weapons will your character use? Will he be a clever scientist or a cunning rouge? What morale decisions will he make? And so on..
You also meet allies along the way, some come with you for a price.. where as others follow you in return for favours, for example, you can save a Dog at a scrapyard and then he will follow you.. you can only have one ally with you at a time, but it dosen't really get that lonely within the wastelands, there's always something to see or a mission to do.
In conclusion the wasteland looks fantastic but it always has that limit in which it becomes repetetive.. especially when your walking far distances across burnt down cities killing mutants and rats for hours at a time, but really it's a huge game and is genuinally as good as everybody says if you put time into it.