Joe - Every MLS has a different format, every single one is different. Once a feed provider steps beyond a single MLS, they will be forced to attempt to standardize the data.
Every MLS also has a set of rules that they stick rigidly to, this adds to the confusion and difficulty (and expense) of merging MLS feeds to a single unified feed either as a data feed supplier or even just to stick listings onto a single site that covers an area with multiple MLS's.
What is needed is a single, country-side MLS format, with a single country-wide feed. This feed would be easy to monetize, easy for 'anyone' to deal with and really easy to 'secure' with digital signatures on images etc. Above all though it would be accurate and complete.
The syndication companies attempt to do this, but they compromise in many, many places. The number of fields available is a major one and this issue plagues IDX suppliers too. A quick count of 50 random MLS's gave me a unique raw field count of over 3300 field names.
Some are simple, easy to deal with dupes, like 'Beds, Bed, Brms, Bedrooms, Bedrs' which can be mapped fairly easily, some are region specific (basement size), some are just plain odd.