It would be great if we could have an aggregate (roll-up) of the internal communications that are being posted to @users on things like notes, tasks, etc. Currently, the user gets an email or MANY emails for these. Also, when they complete them or comment back then the original commentor gets an email. A simple interface to show all of these comments and on what deals, contacts, companies would be great to help manage this. It's a CRM so this makes interacting with the team so much easier than copy and pasting the link to something like a deal in a 3rd party messaging. If you want to make your technology more sticky within an organization then it's wise to get people to spend more time in it.
I believe we're missing the intent of the request which is a "CHATTER" like feature. It's not a report, it's a space where dialog can be exchanged around companies, deals, and contacts. For instance, if a Sales Manager wants to ask Rep A about Deal X, they can do so, but they may also want to ask about Deal A, B, C, D.....so an place where all of that is, where they can then interact with each other, would be ideal. If you're unfamiliar with Salesforce Chatter then I would suggest checking it out here: https://www.salesforce.com/products/chatter/features/
You could use Activities as the primary object and include Deals, Contacts, and Companies. It would take some filtering, but an example table could like this:
Note Body
Contact ID
Company ID
Deal ID
For filters, you can choose to only include Notes where the "@" character is used. And you could add additional columns for any other activity (tasks, etc) where you would be using these.
Alternatively, it might make more sense to do a bar graph broken down by Hubspot user, and then have separate bars for Contacts, Companies, and Deals that you can click into to see the list.