There are many forms of decisions you can make during sidequests. Not all of them affect the following quests. So if you have to kill a monster, you can "decide" to simply go and kill it (quest completed), or do some more research, ask people, examine stuff and then kill the monster prepared with useful stuff (quest completed), OR you find while examinating things deeper you may see that you could complete the quest in a different way. for example if you figure out that the monster isn't really the problem but rather something else. Also there are probably quests where you have to decide to kill someone or don't. At this point you will not know which decision may lead to a follow-up, you have to do what you think is best. Could be you have to deal with a follow-up quest where someone seeks revenge, or you could miss a follow-up quest because the NPC is dead.
i would try to handle it like i did in w2: simplest way isn't the "best" in most cases. Let as many people live as possible - except for those you really hate. Save a lot.
I am not really worried about missing quests or content because of my decisions - as long as i am confident with the decisions i made. I will not let someone/something live because it gives me the "better" reward if i don't want that this someone/something lives. I won't kill anything just because of probably useful drops if i don't think it is the right thing to do and not without checking what i could do instead. I can only be confident with my decisions when i can assume i decided well informed. So i am only worried that i could miss/oversee some important clues, hints or facts during questing.