I would say the overall presentation options are the best additions/changes for 2008 R2.
The indicators are really nice and the charts are SOOOO much better than before. Adding ing Dundas guages and maps is a big plus too. Hopefully mapping will be even better in the next version as I have found substantial limitations with it.
Not really a fan of shared data sets although I understand why they added that feature. For standardized reports where things like drop down lists are constantly being reused then I see the benefit. However, I have always found it more sensible to control this on the back end through stored procs. Then people working in different projects still use the same data sets. Using shared data sets is great for sharing throughout an SSRS project but unless you sink up your project with the rest of the team and share those objects then you run the risk of not using the exact same code. I have also noticed that you have to deploy the shared data sets separately from the rdl which can become cumbersome when you have multiple sets per report.