Without question the number one thing novice designers don't understand is how long it takes to create a good design. Even great designers that have years of muscle-memory can spend hours upon hours on a design.
Novices generally don't appreciate this and just don't take enough time to produce good work.
Replace "novice designer" with "people in general".
I've been having an ongoing fight with a colleague who insists we should redo the design to a completely new theme before launch, with the rationale that it's "ten lines of CSS".
Part of the problem is that one of the qualities of good design is that it looks obvious and effortless. I have spent hours sometimes fine tuning micro-details to get just the right feel. The result of these hours of work is that it looks like it took less time rather than more - the longer I spend, the less time people assume it took :-)
(FWIW, I'm no designer, just a developer who has to put that hat on every now and then).
Novices generally don't appreciate this and just don't take enough time to produce good work.