I just prepared list of features/functionality which are unavailable in BCS. This can be because of technical issues that comes with implementing External Data features and since data is residing in systems which are not in control of SharePoint. Here is the list which you may find useful while designing systems based on EL and be aware of limitations.

1. Workflows cannot be configured on External List.
2. Item level Permissions cannot be given on items in External List interface
3. Export to Excel feature is not available on External Lists
4. Versioning cannot be configured on items in External Lists
5. Item History is not available in External Lists
6. Datasheet View cannot be used in External Lists
7. RSS Feed cannot be configured on External Lists
8. Ratings are not supported on in External Lists
9. ListData.svc can’t be used for providing REST based access to External List data
10. External Data Columns cannot be used as Site Level columns (can’t be consumed in Site Level Content types)