Thanks for pointing out the shortcomings of the "mine green, irrigate brown" advice, Obormot. In addition to understanding the Despotism penalty and what stage of development your city is in (emphasizing growth vs. emphasizing production), one should also consider how corrupt your city is. Distant conquered lands are often only going to provide one shield per turn on their own, and so should often be developed to mazimize food in order to support specialists, whether taxmen, scientists, civil engineers (if you want to build infrastructure in the late game -- say if you want to push for a culture victory, for example) or police (good for getting middlingly corrupt cities back to serious production).
Also, you should think about the limits of how big your city can get. Putting more than 24 food in any city before you get hospitals is wasted. (Of course, producing more than 24 food will help a city get up to 12 pop faster, but they're wasted after that.) So think -- how can I get the most shields (or specialists) out of my 24 food? Is it worth irrigating a grassland if that produces enough excess food that I can now place a worker on a mined mountain? The "mine green etc." advice only gets you average growth/production rather than maximized growth/production.
Also, if you're putting out an odd number of food, eventually you'll put your city in a growth/starvation cycle.
If you read Cracker's article, you'll find all kinds of vital info about the relative value of improved tiles. I never knew how useful mined plains were until I went back and re-read it.