Just an additional note. Current research indicates that surface area is equally as critical (one of the reasons for the slow demise of bio-balls) as oxygenation. The aquaculture industry is largely using fluidized bed filters. While they are not so popular with hobiest, it is clear that the high surface area provided by the fine sand combined with the constant motion of the sand provides for an incredibly efficient bio-filter, despite the fact that it is completely submersed. The fluidized bed filter actually solves some of the problems with wet/dry system which are prone to deadspots (due either to dead colonies of bacteria that became too dry or just areas that are not exposed to enough water). Since the sand particles are constantly banging into each other only the healthiest of the bacteria can remain. Weak, Non-productive bacteria do not have an opportunity to take up space on the surface area as they are soon knocked off the sand particles by this constant motion.
Also, current fully 'wet' systems (e.g., canisters) have available, media which has huge surface areas which sort of offsets the necessity for extensive oxygenation - for example Seachem's Matrix Bio Filter media and Ehiem's Ehifasubstrat.
Also, with the introduction and widespread use of liverock as the ultimate biological filtration the Aquarist has even more opportunities.
"Not everything that can be counted counts, and not everything that counts can be counted."
- Albert Einstein (1879-1955)