Of course, including keywords in the product page title/description and associated feed. But what else?
Would be interested to hear your thoughts on:
EDIT: meant to say - the above are just some things that I'm wondering about, but any other general non-obvious tips also appreciated!
Since asking this question, I've found that uniqueness of the product description seems to be one of the key distinguishing factors. In particular, when a number of vendors have the same description for a product they tend to get lumped together behind a "compare prices" button, making it that much more difficult for a customer to click through directly to the individual vendor's site.
Similar descriptions are common when vendors simply copy the stock description from a manufacturer or distributor's site - it take that bit longer to write your own, but seems to be worth it!
As a rule of thumb I always supply as much data as possible into all the fields available. Note: I don't mean keyword cram, I mean just supply as much detail as you can about the product. This has served me well to date.
Read the official Google Guidelines for more.
I once read an (unverified) article stating that Google shopping does not take the target page into consideration and only the supplied feed. I'm not sure this is true, but even if it were you would still want a well built website with proper titles and content to compete in the standard web search regardless which does mitigate this point.
One thing that is apparent when I search for items using Google Shopping is that the items near the top always have lots of user ratings. I don't think it is a coincidence. It is hard to prove this as in the world of Google nothing is ever confirmed or denied.
PageRank of destination page
Probably no more or no less then regular search results. I wouldn't go chasing PR for this or any other reason. There are much better ways to improve a page's/product's rankings.
how product pricing affects placement
This only comes into play when a user chooses to sort results by price
how age of page affects placement
Irrelevant. Age of a page has absolutely nothing to do with the relevance of a product for search results. If it did new products wouldn't be found for who knows how long and thus the latest cool gadgets wouldn't be reflected accurately in the SERPs.
Images
This is more useful to the user then to the search results. If the product image confirms to the user that you have what they are looking for they are more likely to click on your link.
Inbound links are one of the biggest things that Google looks at. Google treats a link like a vote, sites with the most "votes" have the highest placement. Type the following into the Google search bar link:www.yourdomainnamehere.com example: link:www.facebook.com It will tell you how many inbound links pointing at your site Google recognizes.
Find other sites with similar content and do link exchanges, it's a huge help for high placement.