In the Google & Yahoo search results most of our pages are only indexed by their url. The titles of the pages are not shown nor their descriptions. MSN shows things fine.
Most of the pages are dynamic - created using .htaccess and php. There are no duplicate pages - although some sub-folder names are the same, within different sub-folders. Some categories are empty because our users haven't yet added a tip or a review – so the content of these pages is the same i.e. saying "no review of this item" -although the page title is always unique.
Any thoughts about what the problem might be?
Well, i don't find any problem in this...
You don't have duplicate content, titles, URLs... everything seems fine to me.
Make sure to check does all those things are crawlable by the search engine.