I seem to remember a Brackeys video where he discussed level design, saying how a level should naturally guide the player towards the next checkpoint or exit. You might already know that, but I thought it was a worthwhile tip.
Speaking more generically, realize that the vision for your game is probably going to change with time, so don't get too attached to any particular level design. For example, you might have in mind a level with lots of fancy stuff, only to realize that it causes framerate issues, etc. So I would recommend doing mostly "whitebox" levels to get the basic designs and functionality in place, but at the same time, maybe take a smaller level and really flesh it out just to get a feel for how the "final product" would look.