Any decent CMS is going to take some study. There are some simpler ones for sure, but these have restrictions one way or another.
Many folks just one to download a system, flick a few switches and be up and running. Plug and play if you will. They then want to scroll through a long list of visual templates, pick one and have it work.
MODx really does not fall in that category. Most of the folks here are building custom sites for themselves or for clients. The main attraction is the power and flexibility that programmers and developers love. Everyone who gets involved with more featured CMS platforms have different skills and methods.
One of the key aspects that I do not see highlighted that much is the power and flexibility that MODx offers designers. If you can create good looking and proper functioning sites with clean markup and CSS then adopting MODx is wonderful. There is some jargon and techniques to learn but its all there. Once the documentation gets better I would say the marketing folks really should target the design community. Getting these folks on board will be great - once the core group is ready.
Building that first site can be a chore with some frustration, depending on the skills one brings to the table. The second site gets a little easier. By the forth site you will find that you are learning stuff in great leaps and bounds. Its like a rush of understanding. Especially once you explore how template variables, chunks and snippets go together.
But if you are getting into creating custom presentation and template structure the core skill to have is a thorough knowledge with CSS. CSS is the foundation to everything. Some here value PHP skills higher. Depends on what you want to do of course. While PHP skills are nice to have you can do great things with MODX without it...