Combining RPG with first (or optional third) person shooter, it's runnin' and gunnin' with a point. Gameplay flexibly accommodates the player's personal style, whether playing as a sickeningly polite do-gooder, a snarly kill-everything-that-moves psycho, or shades of grey in between, the game responds accordingly. Actions have consequences and NPCs and quests open up differently depending on the player's past deeds.
The setting is a nuclear-blasted wasteland, presenting a huge, open play area traversable only on foot, though every checkpoint discovered puts a marker on the automap that the player can instantly jump to as long as there is no enemy activity nearby. Walking to discover new checkpoints could be tedious if not for the many opportunities to encounter enemies, stock up supplies and watch and participate in many set-pieces that add interest to the trudge. For example, there's the opportunity to release a Super Mutant Behemoth to see how it will take revenge on its captors (of course, that means having an enraged Behemoth to deal with after) and here was this bunch of Raiders in the act of punishing one of their own -- I tried to rescue the poor guy but he turned on me and I had to shoot him anyway, the ungrateful wretch.
What's fun is that the game world feels like it's living and breathing. There are enemies to kill but there are lots of other people to interact with. A few actually join as a partner -- including a dog by the stupid name of Dogmeat -- depending on their regard of the player. Dogmeat will join up regardless.
Lots of quests to keep the player occupied. Apart from the main quest, there is a long list of side quests, some of which are quite involving in themselves. And it's fun to hear your exploits being reported on the portable radio you carry as "news" reports that depending on your performance will have you praised, villified or questioned by the rather self-righteous sounding DJ.
But it's 3Dog, the DJ, who is the player's constant companion and conscience. The music he plays is far preferable to the "rival" radio station which pumps out insane propaganda messages and Sousa marches. The music 3Dog plays is oddly from the 1940's by artists like Bing Crosby, The Ink Spots, and Roy Brown. My personal favourite track is "Civilization" by The Andrews Sisters with Danny Kaye, a comical and strangely appropriate song for post-apocalyptia as it is today. For the moment I have a bunch of songs older than me playing in my head. They're catchy and the melodies sound quite simple, but I discovered very difficult to sing along with due to certain intricacies in the music that I just don't... get. Weird.
I could go on to discuss the merits of V.A.T.S. and S.P.E.C.I.A.L. (basically an innovative enemy targetting and a character customization system respectively) but they're easier to play with than explain. I will say that I'm a little disappointed that my character is already maxed out at Level 20 and I'm halfway through the main quest only. Yes, there are a ton of rare items to find that can help me increase individual stats and combat survivability but it's a little limiting to think I've reached my maximum potential already when there's so much more left to accomplish. Heh. Sounds like real life.