Good info Celebird.
The seasonal campaigns idea is all well and good, but it's really just increasing the priority.
I think promotions are the least understood (at least by me) yet best way to differentiate yourself. In shopping, it's really all about reviews and price.
Although we are seeing a lot of cross device conversions in shopping.
I'd love to see a lab test result to see the behaviour of those using shopping. It drives me crazy wondering how they sort, if they use the compare tool etc. I wish Google would publish some of that data.