The best thing I can tell you is to be consistent. The impression you first give the user will be what they will expect throughout your site, web-savvy or not. If you change how you input or display data in different parts of your site it will only serve to confuse the user.

For the older crowd that uses just email I would suggest not overloading them with information. Too much information on one screen can be a pain to sift through and usually the younger crowd doesn't mind it as much - mostly because they don't read so much as skim, though - but the older crowd will find it to be a chore.

Users understand the back and forward buttons on a browser. Try not to create a situation where the browser buttons would not function as expected. Too much AJAX or wanton use of a lightbox could be considered such situations.

Lastly, visually offsetting the content of your site versus the layout (menus, headers, footers, etc.) can go a long way to guiding usage of your application.