I think what you’ll find, or at least what I am finding, and what will excite you most, is that the underlying "theme" or template, is little different that any of the sites you’ve already developed. From my also limited CMS exposure, MODx is probably the cleanest for being able to literally drop in your existing design. If you developed with DW templates, you are way on your way (though I’ve tended to avoid them) or even if you just developed a "temp" or dummy page that you used to build a site with, same idea.
Chunks are very much like DW library items... allowing you to display the same content all over and being able to centralize any changes to it.
After that, things get bumped up a few notches based on the dynamic capabilities... Snippets might be compared to php includes and TVs (Template Variables... though I’m still battling to get TV as in television out of my mind everytime I read that

) start to cross all boundaries, from simple to complex and no description I could provide would do justice.
But when you look at your template, drop in a few MODx tags for content, chunks, and TVs, you are still left very much with a full web page from head to... well, body, but it doesn’t get all chopped up, pulled, and stretched trying to skin it over the CMS. From a designer standpoint, this is perhaps the ultimate freedom and makes it easier to transition into than probably most CMS’s. In many ways, if you design primarily from a CSS standpoint versus and XHTML standpoint, you very well might be able to go from Photoshop or Fireworks to MODx, bypassing DW or other editor of choice.
Perhaps the biggest challenge is that once you start to get into MODx you’ll find that your brain starts to run faster with ideas on what to do, what you can do, and how to do it than either your learning curve absorption rate or the development as many of the snippets and MODx is still in the early stages.... though your own limitations will probably keep you busy for awhile before the other issue kicks in.
Luckily, I think the development team is working really hard to keep pushing any limitations of MODx and the snippets further and further out.