I do a lot of mix feedback via Patreon and my mastering service. 

First and foremost, people tend to expect observations about technical mixing and processing. Which is totally relevant of course.

But often the problems I encounter can be traced back to the arrange.

Commonly there is simply too much going on.

The decisions we make with the arrange often affect our mix much more than any technical processing we do.

Some notes:

• Don’t make elements fight for attention and space. Avoid collisions from the get go when possible.
• Fine tune the timing of your elements and how everything plays together. 
• Micro timing and phase – small adjustments in phase can make a big difference in how something sits in a mix.
• The busier the mix, the harder it is to maintain dynamics (and the less loud it can go, if that’s what you want).
• When the mix has contrast and space to breathe, the listener has a better chance to intuitively comprehend what’s going on and take it all in.

In short: 

Arrange in a way that supports the mix. 

Dodge issues by not creating them in the first place.

Then focus on the processing to figure out the rest.