This debate is a bit jumpy, because 1300 Norwegian elo can be ca 1650 Fide and blitz-chess.com and maybe 2000+ online chess.com
what's the main difference between a 1300 and a 1800 player?

Most players make good moves most of the time, then make a mistake. The level and frequency of those mistakes is the difference in most ratings.

Most players make good moves most of the time, then make a mistake. The level and frequency of those mistakes is the difference in most ratings.
I think you are right to a point but, I am not 1800 and I remember being 1300. I would crush my former self easily, nearly every time. I have learned so much since then, in every facet of the game. For me they are nearly like night and day. If you were comparing a 1600- 2100, I think it is a better comparison to what you are saying. A 1300 is too close to being nearly a complete beginner in my opinion. I realize it takes some people longer than others to get to 1300 but, from starting at 1200, I tend to think most people interested in this reach 1300 fast.

Most players make good moves most of the time, then make a mistake. The level and frequency of those mistakes is the difference in most ratings.
You are noting the symptom, not the cause.
The REASON behind these mistakes is not simply blundering. It goes back to what NM Reb said...it is about understanding.

About understanding. I guess 1800 understands that loose pieces drops off better than the 1300 does.

I think things like...
*being able to execute basic tactics versus intermediate to advanced tactics
*understanding where to put your pieces when you open and where not to
*not trading good pieces for bad ones
*knowing when and when not to sacrifice
*the understanding of the value of pieces in various positions and how they can vary as the positions can and or will change
*knowing when you are ahead, drawn or losing, despite material, time, and or positional imbalances
*often the weaker players can't coordinate their pieces and especially are lost without the queen
* understanding of pawn structures from openings, when their preferred opening isn't played
...are all some big differences I see between the two...

Robert0905 wrote:
" also, 1300 frequentlt do not know theory"
In my experience 1300s can often recite line after line of "theory". Many of these can correctly name almost any opening.

ParadoxOfNone wrote:
*not trading good pieces for bad ones
I probably still don't have that down but it took me a year or more of casual chess play here to understand what a good piece vs bad piece is. Thats a hard concept, for me anyway.

Robert0905 wrote:
" also, 1300 frequentlt do not know theory"
In my experience 1300s can often recite line after line of "theory". Many of these can correctly name almost any opening.
LOL I agree with you. Many of them know all the book moves as well. The problem is if you ask them why it is a book move or a play a move that isn't a book move against them. They look like a deer in head lights with that shocked facial expression like "well I don't know what to do now."
Today I feel like 1300fide strenght, And I am almost good in two openings, struggeling with learning two more, and slightly began testing another two.
In 2 of my four latest otb-longchess-losses I met openings from 1600-1700 fideplayers that I wasnt prepared for at all. I was not able to survive and made blunder in one of these games and a positional really bad choice in the other.
I won one game this autumn against a 1815 fide. It was absolutely luck. He played the opening I had rehearsed best, and I had tempo on him at least 10 moves, maybe the whole game. It was mate in 80.
So, there are some 1300 players around that fail when the openings fails, but can win if they come out of the opening with tempo. Getting the opening right automatically sets up a sound development, and then its much easier to win against very much stronger players.
I think one difference is the 1800 is more aware of opponent's ideas and plans while the 1300 just thinks about his own plans and ideas.
I can remember being so focused on offense that I almost neglected defense completely. They tend to telegraph mating attempts. Of course the better your opponent, the more obvious the implications of your moves are to them...