Sales Pitch & Readability
Up until recently (and that made change again in the future), Amazon weighed keywords within the title higher than other input areas.
Now though, all areas are equal, so that you can worry less about keyword indexing and more about convincing customers with argumens and readable titles.
Remember, the title is the first opportunity to generate interest and often it remains the last.
Balance between Main Product Properties & USPs
The title should make it crystal clear what the product offers, as well as what it makes it better than others similar products.
A proper product analysis can help you understand the product's USP, as well as which problem it solves or advantages it creates. To find answers to those questions it is recommended to go through the reviews of the product (as well as competitors), in order to understand which aspects are the most important to customers.
High Influence on Click-Through-Rate (CTR)
Along with the main product image, the title arguably has the biggest influence on the CTR in the searchresults. Again, it can only be stressed that the product's USP should therefore be made very clear in the title.
On a technical side, it is important to realise that the words in the title are clickable. This means a longer title will offer a larger click area, which will generally also lead to a higher CTR.
Limitation of 500 Characters - but varies by Category
Although you have 500 characters available for the title, it is not recommended to use all of thems, as too many keywords can also harm the "readability" of your titles.
For many categories though, you should not use more than 200 or even 50 characters, according to Amazon's guidelines.
Pro Tip: On mobile devices only around 70 of the title's characters will be visible, so make sure to place your most relevant keywords and the biggest selling points at the start of the title.