Someone on /r/adobeillustrator posted an image and asked what parts of the program they should explore next. I ended up giving a list of most of Illustrator’s major features, ranked by how I think about them and how I sort of see them in different levels of complexity:
core stuff: global palette swatches, flat-color shapes with the pencil tool (double-click and play with its settings, the defaults suck), layer organization, draw above/below/inside (and clipping masks as well), transparency. Also of course all the basic shape tools and the pen tool (never turn more than 90° between two points, pull curve handles out to about 1/3 of the length of the curve segment they control, avoid s-curves between two points).
finesse: gradients, blurred shapes, line blends, pathfinder (I almost always hold the alt key when visiting that palette to keep my source paths live), play with bitmap effects and find 2-3 that don’t mostly suck (I find most of them to be kind of ugly, but I love using a soft/hard light mezzotinted rectangle at the top of the layer stack to apply texture to my work, also blurs can be super useful), line width.
don’t repeat yourself: blends, art brushes, scatter brushes, maybe symbols though I rarely use them, pattern fills. each of these is a powerful way to make complicated work quickly; each of them is the right tool for the job sometimes. play with them, learn their quirks, learn when to use one over another.
your invisible assistant: distortion meshes! Make a 1×1 d-mesh, then use the perspective mode of the free transform tool, maybe push the mesh points around a little more, and voila, something complicated in perfect perspective.
advanced: funking up your paths with live effects, layering multiple fills/paths/effects in the appearance palette, saving them for later use. I’ve been doing this a lot lately, as I find ways to make a faux-painterly look that is still clearly not “fake painting”, and renders quickly as my comic book pages get more complicated.
stuff i never use: gradient mesh (too fiddly, doesn’t play well with global swatchs), charts, autotrace.
And maybe that’s the outline for the book on Mastering Illustrator that I’ll probably never write.