As an SF Chowhounder, I come to Chicago about once a month, typically for 3-4 days (dinners) at a time.
I've found this to be my favorite food city in the US, and am amazed at the variety and quality of the food and restaurants you have available.
I usually stay in River North or downtown, and typically don't venture beyond an $8-10 cab ride, and so have explored (I think) most of the good, well known, "casual fine dining" places in River North and River West.
I am starting to feel like I've tried all the good ones, and don't know where to go next. I'm VERY flexible in what I'll eat, and I prefer interesting, fresh ingredients, unusual combinations, etc. My typical price limit (expense account) is about $50 including a glass of wine. I'm often solo, but sometime I can get one of my co-workers from our office to come along.
So here are my favorite places to give an idea of what I like, though I'm not sure what you can tell from this:
1. Avec - probably my current favorite, I eat there EVERY trip at least once
2. Naha - love the food, great service, fresh ingredients and interesting ideas
3. Fox & Obel - my "go to" breakfast place, great (huge) breakfasts and decent coffee
4. Vermilion - I find this to be an interesting place, like the Latin-Indian concept
5. Otom - tried this once, it's a bit clever, along the molecular gastronomy lines
I've primarily relied on Chowhound, and it's been invaluable to locate all of these places and the others I've been to. I could list them all, but perhaps it would be easiest if you have 1-3 suggestions that fit the basic geographic & price constraints.
From a food sensibility point of view, I have found nsxtacy's posts to be very similar to my taste, though some of the places are probably out of my price range (certainly no Alinea, Everest, North Pond, etc.). And I'm not really interested in eating deep dish pizza, hot beef, Hot Doug's, etc. I would welcome BBQ if there is a worthy place in taxi range but most of the well regarded places seem to be too far away.
Hope that's not too vague or confusing. Any ideas would be very welcomed - even if I have been there before, you may remind me of a place I liked but forgot about. Thanks!