Much of the features are listed on their 
site, but below are some that feel like an improvement to current platforms:
- Topic categorization, which allows filtering of posts based on things such as frameworks, general discussion, code questions.
From a user's perspective they aren't posting into a separate channel, just attaching headers to their topic.
 - Social-based
 login - users don't have to sign up to the application and can easily 
just use their own gmail, GitHub, or other social accounts.
 - Modern
 UI. I feel this is a given but there is something to be said when it 
comes to first impressions of a platform. Being told that IRC or lispforum is where people communicate becomes a hard pass for those that are familiar with Modern Web UIs. With the revamp of https://common-lisp.net/ and creation of https://lisp-lang.org/,
 it's at least become apparent that to help grow the Common Lisp 
community there is some change that needs to be made on that front.
 - Open
 Source - and not just that but an ability to gain these items and more 
by just talking to the Discourse team and seeing if they would provide a
 hosted solution for Common Lisp. There are more details on this free 
hosting here. We may be able to reserve https://common-lisp.discourse.group/.
 Should we need to scale up there are discounts for Open source 
projects, as well as a means to perform migrations to a self-hosted 
platform.
 
I assume when you refer to a plan to reach 
"critical mass", you're referring to how we're going to increase 
adoption of this forum. We would need to make modifications to the 
aforementioned 
https://common-lisp.net/ and 
https://lisp-lang.org/
 community pages to include this discourse forum as the forum for 
general Common Lisp discussion, and advertise its creation in the lang 
Google Group/Reddit/IRC.
I do have concerns on 
community splitting, as we'd now have a subreddit, google group (wasn't 
aware of this one), and now a Discourse forum. Though with this 
requirement being posted in the 
contributions needed list I'd initially assumed the pros/cons had been evaluated.
Altogether though, if adoption fails, Discourse will shut down the forum after two-three months of no usage.