The world of fallout 3 is huge. The environment is well designed and makes you believe that you are playing in a post-nuclear landscape. It is really dreary and full of ruin.
There are lots of locations to discover and explore. The character models however, appearance and animation-wise, leaves much to be desired. (Luckily, you never have to look at your own 'face', unless you play him or her in 3rd and you probably won't.) As with Bethesda Soft's Elder Scrolls series, character customization decisions are important to your game and you have lots of options - stats, weapons and skills specializations and perks.
It is a harsh world with very dangerous enemies. It is a challenging game right from the start. It takes a while to get use to the gun combat, and it is pretty good. The VATS system (which is basically for more precise aiming) is pretty good, however it is limited by your action points.
There's lot of things to do. Expect to be swamped in many optional quests within a few hours (like I am right now). I love how one of my first optional quests seem mundane at first and then develops into a really interesting side story.
Update: There aren't that many side quests as compared to the Elder Scroll games, but the side quests are now designed in such a way that accomplishing one goal leads to another. A side quest is not just a simple do-it-and-done thing in this game but an actually well-crafted adventure made of several related quests, one leading to another.
It's going to take a while before one can come to a real conclusion on whether the game is as good as the hype. I have to admit, I was so looking forward to playing this game and now that it is here, I'm really happy and excited about exploring this new Fallout.