Here is what I've found:
Any salesforce field with Data Type greater than "Long Text Area(4096)' will not show in Tableau. Example: Account Description field will not show up in Tableau because that field in Salesforce has a Data Type of Long Text Area(32000).
I don't know if this is a limitation of Tableau or if this is a limiation of Salesforce API.
The workaround: if you need Account Description field, what has worked is create a custom field Long Text Area(4096) which does a look-up of the Account Description field.