Thank you for the kind reply! Yes, we would also love to see more witnesses carry out testing and also report their findings.
Yes, we have mostly been checking for bugs and ensuring that that the new features work well. For the implications of HF21 on things like upvotes and post payouts @jga did a very thorough guide in two parts that you can find here.
Aw, thank you for pointing me at that post! Excellent! I will have to check to see if you are on my witness votes! If not you will be!