At Amazon's size and scale, it's all the more important that you start with something simple with well-understood performance characteristics from day one. AWS doesn't really get a grace period during which they get to fix scalability problems.
AppEngine went the other route and provided a very simple database API at first and all queries had to be range scans over an index. Any query you wanted to perform had to be precalculated by defining a composite index and some things (like inequalities on multiple fields) weren't supported. Over time they've built upon their basic database and added features such as a zigzag merge join algorithm which lets you perform queries that were otherwise impossible with a given set of indexes.
I bet DynamoDB will be going the AppEngine route by starting with a simple, scalable base which can be used to build more advanced query engines and features.