A listing of other users expressing (various degrees of) outrage over the same topic can be found here. As before, the accepted solution doesn't work.
However, one person found a temporary solution using Adblock Plus (here, first message on Jun 26 (of 2013, I assume)):
It's not a problem to do this at all: go to ABP filter preferences, then select "Custom Filters" tab, then "Element Hiding Rules", then click "Add Filter". Now add following eight filters:
mail.google.com##.J-N[title="Social"]
mail.google.com##.J-N[title="Promotions"]
mail.google.com##.J-N[title="Updates"]
mail.google.com##.J-N[title="Forums"]
mail.google.com##.J-LC[title="Social"]
mail.google.com##.J-LC[title="Promotions"]
mail.google.com##.J-LC[title="Updates"]
mail.google.com##.J-LC[title="Forums"]
This seems to work for me. It's really regrettable, since the data is still there, the problem is still there, and this is really just a workaround that may break if Google changes the website's underlying code.
Still, in light of the fact that the "correct" solution to the problem doesn't seem to fix anything, there's not much else I can do. This temporary solution is permanent for me, unless Google can fix the underlying problem.
Best Answer
Like the comments mention, you can't drag and drop into the Primary tab unless you want to always categorize emails from that forum as "Primary"
What you could try doing is setting up a filter that automatically stars an email from the forums email address (e.g. forum-group@example.com). This will also allow you to refine which forum emails are starred (e.g. only contain the words "lorem ipsum" etc)
Once you get it so that Gmail stars email from your forum, click on the Gear menu and select
Configure InboxFrom there you can select the option to add any Starred email to your primary inbox. This should allow you to keep the forum categorization and still have it show up in the Primary tab.