I have working with a client that runs an online store that has been around for just under 10 years and does very well, mainly because it's just a household name in it's home City and has an email database of over 50,000 subscribers, with crazy good open, action and conversion rates from their mail-outs. They also get a fair amount of search engine traffic just through the sheer volume of listings (old listings remain indexable by the search engines) and variety of what they sell.
My question is are there are products or services out there that will allow us to track, over time, the types and style of messages/stimuli a particular return user responds to or avoids? For example, it would be quite useful to be able to track which users respond to humorous email subjects or call to actions on landing pages. Going deeper it could also be potentially of use to know which users are more likely to purchase with a shorter conversion path, over those that don't know what they want and need to be on a page designed for segmentation.
All links sent to a particular subscriber's email address could quite feasibly have a unique identifier in the links included in the regular mail-outs (they do this to a certain extent anyway so that the user's email address is pre-filled for logging in).
I imagine it would be tougher with users coming from search engines and other non-email sources, but even though they represent a decent percentage of overall traffic, they are in the minority, as most users either come directly from the mail-out, or are members, with the majority automatically logging in when visiting.
Any advise would be greatly appreciated and hopefully it's the sort of thing that many of our fellow marketers could benefit from.