I took the time just now to go through and draw out most of the common plant deficiencies in pictoral form. This diagram is by no means an exhaustive reference for all deficiencies and certain plants vary in their symptoms, but in general most deficiencies will look something like the diagram below.
There is a red dotted line that shows deficiencies that show up in new growth and deficiencies that show up in older growth, this is an important line and attention should be paid to it!!
Hope this helps people identify what they have a bit better. ***Edit***
This was updated as of 2016 and is correct to the best of my knowledge. Iron deficiency is seen in the new leaves only.
THE PHOTO BELOW IS AN OLD VERSION (Notice the red line was shifted downwards, meaning iron deficiency occurs only in the new leaves - NOT the entire plant on most aquatic species).