Great question. All discussions are searchable using the Global Search. Just click the Navigation (or use Alt+N) and start typing. You can't search by date yet. I don't believe you can search based on the date yet from the Global Search, but you can create a report for Discussion messages and put this as a sort or filter.
There is no default consolidated thread, but you can create this by making a customization that takes all discussion posts on tasks and copies them to the project level as well. If you hop on any Advanced Q&A webinar, a CSM can show you how to build this out ( http://www.clarizen.com/live-demo.html)