One of the (many!) things I am working on is a set of pages for each game. For each one I want to list some of the key changes and updates in each game. Something similar to the right hand column of http://pokemondb.net/black-white
For example in Diamond/Pearl, we would put that they changed the physical/special move categorisation to be for individual moves, not based on the move type. And in Ruby/Sapphire double battles were introduced.
Anyone who wants to help, simply post an answer with some of the changes. You can list as many or a few as you like. Try not to repeat what others said. Huge thanks in advance to anyone who helps!!