So that seems fairly straightforward - what's the problem. Water depth - wetlands can range from wet to dry, some are continually under water, others have seasonal surface water, some are saturated but never have water at the surface. Wetlands are often transitional - between dry uplands and deep water habitats. This idea of scale can complicate things. We often like to define based on scale.
Location is highly variable.
Some are in lowlands, some on sideslopes, some are in mountains, some by oceans. Again the natural variability is large. How wetlands look is influenced by location as well and human influence. Wetlands are not all pristine environments, untouched by human hands. Many have been disturbed thus altering some of the basic characteristics.