Might be its too late to get this posted but then I want to do it because I would love to have these handy. This huge list of non-availed features in External Lists via (BCS) will be helpful to understand the limitation of External lists in SharePoint
1) Workflows cannot be configured on External Lists
2) No Event Handlers on External Lists
3) No Versioning supported and so there no history
4) Datasheet view not supported
5) Inline editing not supported
6) Export to different office apps like excel or project or visio or access are not supported
7) Information Management Policies are not supported.
8) Attachments are disabled.
9) Ratings not available
10) Item level permission not supported.
11) REST access through svc(ListData.svc) are not supported.
12) RSS not available
13) Validations are item level or field levels are not supported.
14) Lookup columns for items except for (ID column) are not available