Using Splitly, you are able to automate the testing of the following fields (rank in the order of which items are most likely to be successful in terms of split testing):
1: Price (notably Sale price followed by Standard Price)
2: Main image (but you can also test all images, including the order)
3: Product title (but only if you can update your title)
4: All features/bullets, including the order
5: Product Description
Note that to test these fields, you must have listing control for each. In some instances, sellers are unbeknown to not have listing control over specific fields. In this case test rotations will fail. Most of the time sellers have full listing control so it's usually not a problem. The most restricted field is the product title. Sellers almost always have full listing control over pricing.
The best thing to do is to run a test and we'll see which fields can and cannot be tested on your products.
At this moment in time it is not possible to test backend keywords, however, this is currently in the works.