Well, I've spent hundreds of hours on this game without mods, with few mods, and with many dozens of mods, so I feel I can provide some input on that question: The game is great as-is without mods. My latest playthrough is actually vanilla and I have to say, it's just as much fun as with mods, but with less hassle (and that's using Steam's workshop, and as much as I dislike Valve/Steam, the workshop is incredibly neat). Don't get me wrong, there's some brilliant mods (e.g. dubs hygiene, or the mods to make medical stuff more complex), but they're certainly not necessary. And as the others have noted already, it's not like the Steam workshop is the only source for mods, is it ;)
