Guys, all this is being discussed with the Plant Finder, we will change the description to hardiness. Anyways, this is my interpretation of difficulty and the description I used for this site's database:
Hardiness (will read Difficulty): Each plant is classified under one of the following categories based on its hardiness:
Very easy: nearly indestructable, requires little to no special attention, grows even under standard strip lighting <-- Hygrophila polysperma, Anubias barteri var. nana are good examples...plants you can toss in a 55g with standard 80w lighting and still grow in quartz gravel.
Easy: easy to grow, although perhaps requiring a richer substrate or more lighting than the above <--Rotala rotundifolia, Hygrophila difformis, Bacopa caroliniana are good examples, basically your stem plants that need more light than the very easy plants, but are still relatively undemanding.
Medium: not too difficult if the hobbyist has ample lighting and uses CO2 injection <--Glossostigma elatinoides, Cyperus helferi, Hottonia palustris are good examples, basically including those plants that at least need good lighting and CO2 to do well. Some knowledge of fertilization required.
Difficult: plants that require ample lighting, CO2, a standard, balanced fertilization regimen, and/or special substrate needs <--Rotala macrandra, Ludwigia 'Pantanal', Cabomba furcata are good examples of these plants, which absolutely need CO2 and a good working knowledge of fertilization through the water column or substrate to succeed. If you don't know anything about adding nitrates, phosphates, iron, etc to your tank, then you should probably stay away from these.
Very difficult: very sensitive, requiring a very stable, balanced fertilization regime and/or requires special conditions such as soft water <--Tonina 'Belem', Nesaea sp. 'Red Leaved', Eriocaulon sp. 'Mato Grosso', some rare crypts...in this category, we've included all the plants that either prefer relatively soft water (GH 5 or less), are really sensitive to nutrient fluctuations, or are difficult to get going in submersed culture (C. nurii, griffithii, tonkinensis). Basically, the plants you have to coddle.
Hope this is clear,
Carlos