The game mechanics of Fallout 3 are so similar to The Elder Scrolls 4:
Oblivion that some people have called the game "the best Oblivion mod
ever made". Of course it's not an Oblivion mod because, while it uses the
same game engine, it has been clearly improved. But the game mechanics are
indeed very similar.
Minor spoilers might follow.
Like in Oblivion, the graphics are quite good. On the other hand, since the
game is set in a post-apocalyptic world destroyed by a nuclear war, the
scenery is not really as beautiful as in Oblivion. However, the deserted
landscape and destroyed buildings have their own coolness factor.
Also like in Oblivion, there are basically no boundaries. If you want to
just take a random direction and start walking, just to see what's there,
you can do so. Like in Oblivion, the outworld is huge and completely free
to be explored. However, there's perhaps slightly less variety because it
is, after all, a post-apocalyptic wasteland with destroyed buildings, highways
and cities here and there. There is quite a lot of rich detail, though, and
very little repetition, so there is quite a lot to explore in the outworld.
The Fallout 3 engine clearly improves over Oblivion in that there are no
loading pauses. You can just walk and walk all you like, from one end of
the enormous sandbox to the other, without seeing a single "loading" message
or pause. The engine loads everything transparently and smoothly without any
interruptions.
Character animation, especially facial animation and body language, while
still a bit stiff, is better than in Oblivion. Voice acting is good as well.
Sometimes some situations can be solved in more than one way, which is nice.
For example, you could just try to kill everybody and get what you want, or
you could try to talk yourself out of the situation, or perhaps make a small
sidequest to get what you need. The game doesn't always force you to do
something in a specific way. Some of the solutions may be more difficult
than others, of course, but they may also be more rewarding (eg. in that
they get you more exp).
Like Oblivion, the game starts quite hard (after the introductory levels)
and gets easier as you progress. Unlike in Oblivion, enemies don't seem
to get tougher as you progress, so the effect is even more pronounced.
The face creation stage at the beginning of the game (similar to the one in
Oblivion) is quite fun to play with. On the other hand...
regardless of being able to carefully fine-tune a face for your character,
you don't get to see this face basically ever again. Even when switching to
third-person perspective, the playable character always looks in the same
direction as the camera, which means that you only see the back of the
character's head, never the face. In Oblivion you could at least see your
face in the inventory screen, but not in Fallout 3. (You can sometimes see
glimpses of the face when sitting down on a chair or in the VATS slow-mo,
but just glimpses.)
The main quest felt a bit short, and it ended a bit unexpectedly. Also,
unlike in Oblivion, you can't continue playing after the main quest ends.
It's just that, the end. Thus if you wanted to complete any sidequests,
you would have to load an older save (or know to complete them before
doing the last part of the main quest).
Like in oblivion, no puzzles to solve, and not much of exploration in the
quests themselves (even though the outworld is completely free to be
explored).
Also like in oblivion, NPCs and sidekicks sometimes tend to get in your
direct line of fire way too easily. In this game there doesn't seem to be
any hugely negative consequences (besides lost karma), but it's still
annoying.
There is an enormous amount of content in this game which you don't get to
see if you play the main story in a straightforward way, and not only in the
form of (more or less independent) sidequests, but different ways of performing
the main story. For example, there are at least a half dozen sidekicks you
can potentially "hire" as your companions, assuming you encounter them in the
first place, and then do the proper things to get them join you (when I played
the game I played the main story through without getting even one of the
sidekicks, not even the dog, which I assume should be hard to miss, as some
FMVs assume you have the dog as your sidekick; I somehow managed to miss it
completely as I didn't see any dogs anywhere when I played the game).
This is a two-fold feature of the game. On one hand it makes it feel more like a role-playing game and less like a railroad because you have a lot more choice and you can complete the game in many different ways, which is cool. It also increases replay value.
On the other hand, not all people want to play a game many times simply
in order to see all the content. Missing all the optional content made the
main story very short, even though it could have been much longer, and it
might not be very tempting to play the same game again just to see that
optional content.
The game still suffers from some instability bugs even after numerous patches
having been released for it. In my case the game became unstable after walking
long enough on the outworld: After having walked a long-enough distance, the
game starts first lagging here and there, with small pauses which get worse
and worse, after which the game inevitably crashes. Fortunately just restarting
the game reverts it to a stable state where it's again possible to walk the
outworld for a rather long distance before the instability kicking in again.
The game doesn't seem to suffer from any instability when playing in indoors
scenery.
The sounds are also somehow buggy. Sometimes part of NPC dialogue is just
skipped, ie. not played at all. I read a tip from somewhere that lowering
the sound hardware acceleration to "basic" in the Windows control panel
fixes the problem. It indeed fixes it, but it results in another problem:
At times (fortunately quite rarely) the game just starts producing really
loud noise, at full volume.