I’ve played Fallout 3 and Demon’s Souls, and I love them both right to death, but they are very different things. Fallout 3 isn’t even a little bit challenging- the idea is for you to find a unique way to experience the content, but whether or not you are capable of making it through the content is never in doubt.
With Demon’s Souls, what you’re talking about with the ‘tedious and overcautious’ is precisely what makes the game what it is. If you’ve played the first Resident Evil, it would remind you of that. Actually having to plan in advance how to deal with a threat that’s around the corner, backing off when you think something ahead is too much for you, because it’s smarter to take your loot back to base than to risk losing it all when you die…
…that’s right. In Demon’s Souls, you collect souls (ala Devil May Cry or God of War) from everything you kill, and you use this to upgrade your weapons, learn spells, improve your stats, and so on. If you get killed (and you will), you lose every unspent soul you have. These souls appear on your bloodstain where you died. If you can fight your way back to it from the beginning of the level, you can reclaim those souls. But if you die AGAIN, a new bloodstain forms at your new point of death, and the old stain (along with the souls there) are gone forever.
Also, when you die, the place where you died may become harder. Warning you now- if you spam yourself in an area you have no business exploring yet, the game will punish you. There’s 5 worlds, and I had myself locked out of one of them for a week because I made that mistake.
That’s why the save system is so important. In such a system, you’d be tempted to cheat- get an autosave or save point every time you’re about to do everything challenging. That’s precisely what ruins just about every game that claims to be hard- even Ninja Gaiden Black, challenging as it was, just lets you spam yourself thoughtlessly, over and over, at a target until you eventually win. DEmon’s Souls is innovative- it autosaves…constantly. All the time, no matter what. You can just get up and shut the game off whenever, and when you start again, there you are. Of course, this means that when you die and lose a bunch of souls, you REALLY lost them. No backups, no previous save states, none of that cheating that you forgot was cheating because every game lets you. If you use a healing herb, that’s a healing herb you’ll never see again. If you use them all to beat a certain boss, you don’t have anymore until you find or buy some- you can’t go back and do the boss again once you learned the ‘trick’ to make sure you have more items.
Fallout 3 was fun, but I never once (not once!) found myself jumping out of my chair to point and swear at the TV, and shout “In your face!” when I beat a boss. If you make it past the first level of Demon’s Souls, you’ll feel like you accomplished something.
Demon’s Souls also has far, far more character customization than Fallout 3, in the sense that no matter what you ‘choose’ in Fallout, by the time you get to the end of the game, you’re awesome at everything anyway, so it doesn’t matter. In Demon’s Souls, customization choices really affect how difficult some content will be for you, and certain things that you might or might not experience otherwise.
Also, and I can’t stress this enough, you actually have to learn to be good in Demon’s Souls. That’s something I noticed is lacking in God of War- as fun as that game is- it has you leaping all around spamming attacks that you don’t even know, into crowds of enemies, without paying attention to your life meter. And that’s on hard. In Demon’s Souls, you’ll wind up knowing exactly how long your weapon is, what each attack does, when it is and isn’t useful, when you should hold it in two hands vs. one, and if you can interrupt the monster’s slam attack with a stab before he squashes you, or if you should roll away instead (and which way to roll so you’re in position for a counter). If you don’t know these things, you’ll just get your ass kicked forever and ever, and have to play a different game. You’ll come out of it surprised at how bad at video games you used to be. 
So yeah, the major difference is Fallout 3 is like Disney World, doing everything it can to make sure everybody has a good time. Demon’s Souls is like climbing a mountain- it’s certainly rewarding, but it’s there to ASK if you can climb it, not to ensure you can by design.
