Oblivion is Fallout 3 without Guns/ Fallout 3 is Oblivion without Bows, Axes, Maces, Magic. Both have swords, oddly enough.
That's the closest comparison, and a lot of that has to do with the fact that they both use the same engine.
Morrowind, being the game before Oblivion, is also similar in playstyle and the like to Fallout 3. Moreso than WoW, at least.
You may have people call Fallout 3 a FPS. This is mostly incorrect, as FPS implies twitchy reflexes, and near perfect aim. Fallout 3 does not require twitchy reflexes (unless you're playing it that way) and without leveling up your various projectile skills (Small Guns, Big Guns, Energy Weapons), your aim is less than stellar. Of course, as the saying goes, close only counts in horseshoes and hand grenades (explosives skill), so... Hand Grenades. Use'em. Most FPS games do not have a system in which you can pause the action indefinitely while deciding on your course of action (usually 1-5 shots, depending on the weapon in hand), then have said course play out in slow motion. Fallout (and by extension Fallout 3) does.
If you pressed me to compare it to a game without you, the player, having played Oblivion/Morrowind/Deus Ex/System Shock, then... I suppose I'd have to compare it to Bioshock or Half Life 2, in that Fallout 3 is a story-based First Person game (with a 3rd person option, if you want) in which you, the protagonist, move about the game world and achieve various goals. The comparisons fall apart as the world itself is not level/stage based, unless you were being fairly generous with your notions of Level/Stage and were counting everything not in a building or cavern system to be an Overworld in which you moved about and selected your stages at your whim... but in that case it'd be an Overworld in which quite a few story-specific events happen, a majority of the game takes place in, and quite a bit of your combat as well..... really, it's not an Overworld. Not unless you're just using an extremely basic definition set.
It also falls apart as neither Bioshock nor Half Life 2 have dozens of NPCs with often unique dialog options. I'd mention the morality system as well, but.. it's a fairly simplistic one. You can game it to your advantage pretty easily. Be as big of a murdering jerk as you want, but just hand over some purified water to some beggars for an hour, one bottle at a time, and suddenly you're a paragon of virtue. Even if you nuked Megaton. This also leads to some hilarious dialog from Three Dog over the radio. (*Depressed, spiteful voice* "The Scourge of the Wastelands is at it again. Let's see just how the Lone Wanderer from 101 is fucking us today....*overexcited, jubilant voice* Woo hoo! The Lone Wanderer led a daring rescue of some kidnapped citizens of Bigtown. Good job, 101! We need more like you out in the Wasteland.")
The game is fairly open. The primary quest of the game starting out is to Locate your Dad, and the only real lead you have is.. well, nothing, really. Assuming you follow the only real signs of civilization you can see from the starting area, you end up in Megaton and find out your Dad's gone into the DC ruins and blah blah blah. OR.. if you already have done that, you can just.. go to where he is, and skip about 15 quests. Sidequests can be taken at any time, and a lot of them are of the "Oh, hey, when you get a chance, do this thing" variety, so you can quickly accumulate 20 or more quests and accomplish several at once while you're heading to a location, as you're just doing them as you go along. Most of the quests are chains, though, and tend to start with "Go here and look at this thing" type stuff that later branches into whatever the quest is. There's also a couple dozen or so hidden quests that never show up in your journal and you kinda have to look for them - these are usually single or two stage quests because of their nature and the fact that they don't show up on a quest log, so if you don't do them right then, you'll likely forget about it. Jiggs' Loot is a perfect example of this - you have to input the correct number at several terminals in the Museum of Science. If you do this correctly, you get a note about the person the message was for should meet the message leaver at the usual spot - a diner out in the middle of nowhere. Whenever you go there, you find the message leaver dead (the body isn't there if you haven't gotten the note, and you can't get the note without putting in the right codes) and a couple of goons jump you. Among the loot is a unique Chinese Assault Rifle.
None of this shows up in your quest log, though the note does show up in your note log.
Anyway, hope some of that helps.
is too progressive for you, that's how science identifies you as an earlier species" - Luke McKinney, Cracked.com
Honestly, if you're talking BBQ and 'a guy in a parking lot' isn't part of the conversation, something's wrong."