No joins - SimpleDB was designed for parallel operations - so if you need data from multiple tables, you need to fire multiple queries (in parallel preferably).
No retrieval of multiple attributes - You cannot retrieve attributes from multiple items. So if you need to retrieve 10 items, you'll have to first fire the condition query to get the item-ids and then 10 queries for each item. If you're using serial access, this could be painful.
UTF-8 issues with the REST interface - Minor annoyance. It works with the SOAP interface though.
1024 byte limit for each attribute - This could sometimes be an problem, especially if you're using UTF-8 data (with the REST interface issue)
If you're ready to code for parallel access, no UTF-8 (while using REST), and don't mind the 1024 byte limit then you could give it a try.