I spent three days painting the baseboards in my guest bedroom last month. I bought a standard gallon of semi-gloss white paint from the hardware store. It was on sale for twenty five dollars. I thought white paint was just white paint. I was completely wrong.
The cheap paint dried exactly the way I brushed it on. You can see every single brush stroke and bristle mark in the finish. I tried sanding the baseboards down between coats. I tried adding a liquid conditioner to slow down the drying time so the paint could settle. Nothing worked. The baseboards look like they were painted with a stiff kitchen broom.
I eventually bought a gallon of expensive urethane trim enamel for the hallway. It costs almost three times as much per can. It is entirely worth the money. It levels out completely flat while it dries. You just brush it on and walk away. The final finish looks like it was sprayed on in a factory.
If you are painting trim or cabinets, just buy the expensive enamel. You cannot fix bad brush marks once they dry unless you want to spend an entire week sanding them completely off the wood.