So basically EAX 3.0 introduced new (and to your credit a lot better) reveb effects. EAX 4.0 allowed for multiple reverb effects (Creative calls them environmental effects) simultaneously (also it added new reverb effects, as does every new EAX version). EAX 5 added Environment Occlusion, which according to Creative, "recreates the effect of sound from adjacent environments passing through solid objects". EAX 5 also allowed for even more multiple reverb effects to happen at the same time (and, of course, new different reverb effects).
I agree there is a large difference between EAX HD and EAX 1/2, but even EAX HD boils down to reverb effects.
Perhaps it would be better if we clarified what a "reverb" is:
http://audacity.sourceforge.net/manual-1.2/effects_reverb.html