There are a number of features that are missing when you create an External List based on an External Content Type. This can cause a lot of issues with end users because even though it looks exactly like a normal list it might not behave exactly like a normal list (or at least have some features missing)
1) Workflows cannot be configured on External Lists
Unfortunately as you can see you cannot actually run a workflow on an external list. You neither have the option to define any workflows nor can you actually start one. Now this is even if the workflow that you are thinking of defining will not alter the External List item in anyway, doesn't matter, External List = No Workflows!
2) Cannot create Information Management Policies
This kind of makes sense. I mean you can really declare something a record or move something to another location if you aren't storing it within SharePoint but it is worth knowing. No IMP's means no barcodes, auditing, retention or labels.
3) No versioning or version history
You cannot version or see version history for an External List. Now this does make sense since in many external systems there is no concept of versions
4) No Inline Editing or Datasheet View
Inline editing allows you to edit list items from within the list view itself, kind of a datasheet view for an induvidual item. Datasheet view is a way where you can easily change metadata for multiple items. Neither of these are available.
5) No ratings
This does suck but you cannot rate any items in an external list.
6) No ability to Export to Excel, Create Visio Diagram, Open with Access or Open with Project
Some of these are not available to all list types (Create Visio Diagram or Open with Project for example) but all standard lists are able to be exported to Excel or opened with Access. This is a real shame I think because exporting to Excel is an oft use feature that will be missed by users.
7) No REST access through ListData.svc to External Lists
Yep you cant access an External List though REST...
8 ) No RSS Feeds
No RSS Feeds are available either
9) No Item Level Permissions
You also cannot configure item level permissions for External Lists.
10) No item or field level validation
So as you can see there are quite a few limitations that one should be aware of. The thing that is concerning is that from a users perspective you cant actually tell that a list is external and without all these things being available this can lead to some confusion. But it is out of the box functionality after all and with a little SharePoint Designer and XSL magic you might not be in the situation to expose external lists anyway.
11) No Lookups
Unfortunatly the only thing that you can do a look up on in an external list is on the ID column, anything else and you are out of luck.
12) No attachments
No attachments either!