It's true that Discussions looks quite a bit different than the rest of the wiki. It has lots of open white space and one central feed, much like Twitter. The reason for that is that it was built with mobile devices in mind, unlike the rest of the wiki, and on mobile, this kind of layout works best. Lots of platforms on the web do something similar, for the same reason.
The old forums looked and worked great on desktop, but they have quite a few weak spots, some of which you can see as a user (it doesn't work on many mobile devices and can't be searched properly) and some of which only our engineers can see (the forums are very prone to bugs that are really hard to fix, making them a feature that is impossible to maintain long-term). That's why we built Discussions to replace them, and why you can't enable forums on wikis anymore. There are still the very old DPL forums that wikis can set up on their own and use instead: community.wikia.com/wiki/Help:Wiki-style_forums
Discussions is still being worked on, so it's still lacking a number of features we agree it should have, for example more ways to customize it. Other features we still want to add is a way to notify users about important posts (like you can highlight threads in the forums) and text formatting. Since those directly influence how users use the platform, we're focusing on those first.
We deliberately decided not to have Discussions activity show on Wiki Activity, for these reasons:
* On active communities, there tend to be A LOT of Discussions posts. Sometimes hundreds per day. They'd overwhelm the recent changes feed and crowd out wiki article changes.
* Discussions can't be vandalized, since everyone can only edit their own posts, and they are opinions, not facts, like articles are. So it's not necessary for admins to monitor everything that is going on. Plus: Users can report problematic posts, so admins can see at a glance where their attention is needed, without reading everything.
By the way: If you want to set up a Recent-Changes-like feed that shows you all the activity in Discussions, you can do that. There is a user-generated script one of our helpers created just for that: dev.wikia.com/wiki/Discussions_Feed