Discus in fact are one of the hardiest fish I have kept. It isn't that they are difficult to keep, more that they require more care and even more than that, more knowledge.
That is 100% right, Discus are cichlids and by nature cichlids prove to be hardy fish and for the most part Discus are very hardy. I have seen Discus who were very thin with a condition known as "razor back" caused from internal parasites and worms, and after several weeks of treatment they regained thier normal apperance and were eating vigerously.
As Brian stated Discus come from the "Blackwater" regions of the Amazon. These regions do contain clean waters, but the water is dark in tannic acids and very low in
ph somewhat because of all the rotting vegetation these waters contains. Jack Wattley, Dr. Axelrod and others went through tremedous pains to bring us the modern Discus hobby.
One reason that Discus keeping requires regular water changes is because if you are keeping sub-juveniles with juveniles or juveniles with adults the fish secrete a hormone that impededs the growth of the smaller fish. Pictures on the internet, in magazines and in books can show you what an impeded, improperly fed and wrongly kept Discus can look like - Not Pretty.
Nutrition is a big part in Discus keeping success so is the prevention and treatment of Hexamita and Internal Worms. Also Head and Lateral Line Erosion occurs in Discus but again with proper water quality and regular maintenace these issues should remain at a big minumum.
When I came to AA asking about Discus I was doing bi-daily water changes of 30% on my 80 gallon Discus tank. Brian recommended I start doing daily water changes which at first made me concerned I would shock my Bio-Filter. I said what the hay and gave it a shot. Not only has my water remained cleaner the growth level of my fish has greatly improved. Thanks again for that tremendous advice Brian!