I know the major differentiator for this game is that it is online, but adding random people from the internet to anything has the potential to turn it into a toxic wasteland. Even if they're not being intentionally toxic, other people can be annoying. Once I was trying to listen to a holotape for a quest, and these two nearby guys wouldn't shut up, and I couldn't hear it. Another time I was in a building trying to use a terminal for a quest, and some dude kited a high level super mutant into the room. I had to drop what I was doing, and just make a run for it.

There's also the threat of unwanted PVP. Once you reach level 5, you can be attacked by other players, and if they kill you, they can loot all your "junk". Meaning, all your building and crafting supplies. They can't take your armor, weapons, or other usable items. But still... the nagging fear that some high level dude might pop out at any moment and ruin my day isn't the best feeling. Credit to Bethesda, though, for proactively creating systems to help prevent abusive behavior, like being able to block someone from your session.

At one point, a random guy walked up to me, and just said "I'm going to follow you now", and I thought "oh great" very sarcastically. I was level 4, almost level 5, and he was level 8. I kinda just wanted to play by myself, and I mostly expected he just wanted to murder me as soon as I hit level 5. However, despite my fears, he turned out to be a pretty okay guy. After stashing all my junk at camp just to be safe, we went and did some exploring together and it was kind of neat to be playing Fallout cooperatively. So maybe other people aren't all bad. By the end of my play time, I had decided to try not to worry about what other players might do, and just play the game how I wanted.