I repeatedly asked for calendar custom fields prior to and throughout the beta phase, but despite general support the feedback from IPS was that they didn't foresee the need. Shame really as they would be good for all sorts of uses.
I once had a slow query issue caused by the indexes of ibf_reputation_index becoming disabled which interestingly did not show up in the ACP index checker. Different version I know but as @Kirito suggests a check of the indexes might be worthwhile.
I really like the fact that search in 4 returns all results irrespective of which part of the site you are viewing. Most of my users at least are looking for any and all info on a topic and to expect them to flit between pages and articles and database records is unreasonable.
Can you say a bit more about how this works responsively. One of the much requested suite features is the ability to automatically re-size images on upload. Can this system do this and will it automatically resize an image (or serve a different sized cached image) depending on the type of device making the request?
One question: I'm a little surprised that the sidebar is on the left hand side of the page as this is usually the spot for primary content rather than sidebar stuff. Can the sidebar easily be moved to the right hand side of the page? (I'm sure that you can move it with a template edit, but a nice easy setting would be good please.)
and one thought: An obvious extension of this is to allow registered users to setup their own sidebar in a similar manner to customising their profile area. Is this something that you're looking at or is it likely to be too resource intensive?