The main thing that has changed is blocks and topics.
If your plugin references these then most likely it will need to be modified to work in Geeklog 2.0.0.
Most plugins if they needed a block before Geeklog 2.0.0 they would just create one in the blocks table. This is not ideal since things can change. Take a look at dynamic blocks in the wiki and for an example the Calendar plugin was converted to use a dynamic block for the Events block.
In regards to Topics the Staticpage plugin is a good example. It is assigned a topic if it is a center block. The Staticpage plugin included with 2.0.0 has been updated to use the new topic control which other plugins can use as well by calling the following funcitons. Articles and Blocks use the same Topic control as well.