DD4T-2 java is not backwards compatible with DD4T-1. The root cause is that the published XML has been updated; you would have to republish all your content. We've also changed deserialization frameworks, so you have to update several jarfiles. Finally, some internals have also been updated - the dispatcher-servlet and url-mapping xml files will be updating.
For reference, please checkout the dd4t-example-site in dd4t-2; its essentially the dd4t-1 example that's been fully updated and cleaned.
If you use DCP's, dd4t-compatibility would give you the old componentFactory API. You may want to consider using dd4t-databind if you want to take advantage of strongly typed models.