The combat is very rudimentary at first, but gets better once you receive more skills.
I suggest you do - every area has at least a "main story" of sorts that is enjoyable, even if the rest of the content may be lacking, depending on the area.
Having said all that, the main story skips many areas altogether if you don't specifically unlock them.
Even on Nightmare you really don't want (or need even) to get them all.
The above applies especially to the "Shards", they are like the biggest scam ever (the "payoff" from getting all of them was so insulting).
You don't need to pick up everything (although there's never too much Elfroot).
Generally, if you have OCD tendencies, try to tone it down and you'll enjoy the game more.
Unless grinding for levels when playing on Nightmare, you can safely skip it.
You can pretty quickly tell if a quest is just of the fetchy variety.
However, the game being called a single-player MMO is a fair criticism, so just some points: As someone who admittedly is very much fan of the series (yes, that even includes 2, although $DEITY knows it has lots of issues - but let's not go there now) I might not be the most impartial commenter, but sure, especially at that price.