The Discourse feature I hate the most is that neither New nor Unread show me everything I have not read. Also the counts associated with them are almost invariably wrong. My workaround is to flip the forums URL to include /unseen which on pressing return gives me the list of what I have not seen. (And yes I know that that list can be long but in some downtime a while ago I went through and “read” everything that unseen brought forth.)
The example of how this can be done is on the NaNoWriMo forums with an explicit link for unseen.
I’ve added this as a category to the main navigation menu, top right, to make it a little easier to get to than having to type it in.
However I don’t see any obvious settings for adjusting what gets shown in your Preferences: Interface: Default Home Page setting, though. The NaNo forum is likely using a plug-in to add that, or been custom coded to do so (it is rather highly modified in general). I briefly looked into it, and there is nothing official, just a volunteer made plugin with a warning that it uses an unofficial hack to work, and it might break in any update. So I don’t know…
You can of course bookmark the /unseen URL in your browser and otherwise use the navigation menu when you want to return to it from within the site (that menu by the way is bound to the = key all by itself).
I use Unseen all the time on every Discourse system where my accounts are. It doesn’t need to have a preference because the counts for New and Unread are rarely correct on any Discourse server. Unseen does what it says; shows unseen entries no matter how long ago they posted.