To my mind, writing simply and writing descriptively aren't mutually exclusive. A piece can be comprised mainly of very simple words in a relatively simple grammatical structure and yet be evocative.
Of course, without having comfortable access to a large vocabulary one risks generating page upon page of trite and repetitive phrases that make for a painful reading experience. I never use a thesaurus, but I think one could come in pretty handy to a professional writer - as would a dictionary. But an enumeration of all the words one knows is not the same thing as one's working or functional vocabulary.
Getting the word a day from this site and from merriam webster is a good way to gently stress one's vocabulary neurons, but I also do the tried and true thing of noting words and passages I don't understand when I read so I can look them up later and ponder over them. It's not a quick fix, of course, but over time, it really helps. Also reading a wide variety of things can help.
You might also try to develop exercises for yourself. For example, pick a topic and try to write everything you can about that topic and then hone what you've written into a coherent essay.
Since I'm not a writer you should not take anything I say seriously. I think you're in the right place here to work on your vocabulary and get insights into meanings. If you're interested in improving the style of your writing, you might want to check, as well, with usenet groups like misc.writing. Haven't checked in with them in a few years, but maybe they can give you ideas for exercises - or perhaps even suggest a regimen for developing specific skills.
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