Looking at the marketing material for Salesforce, especially the edition comparisons, the differences between the Salesforce versions are very high level. However there are a few key things missing in Group Edition that make it very limited compared to Professional edition, and unless you are selling Widgets, there is not much in Professional edition that makes it worthwhile. The only edition that is really worthwhile is Enterprise, but the price is usually too high for small business.
I moved the rest of my question into an answer to make it a community Wiki Post