Pet peeve: People who complain excessively about adverts.
If you've ever owned a website that provided some kind of popular service you're probably familiar with the cost involved. Registration, hosting, software licenses... It can start to pile up pretty quickly, and that's if you're planning on doing everything yourself. You may also need to shell out elsewhere to make up for any skills you lack, buying templates, stock images, plug-ins, and so on.
Then there's the endless maintenance involved. At the very least you need to keep all of your software patched up, take backups, and keep up with possible security issues. You may also have to try and recruit volunteers or even paid staff. The sheer amount of time involved can be the biggest killer.
Unfortunately, the internet has a culture of consumption. There is this general feeling of entitlement to content which doesn't accommodate for the producers. The internet does not run on pixie dust. Like many of the best things in life, it runs on cash.
They really do. As a discussion platform they are fundamentally flawed, and in this era of 'social media' they feel more and more like a clunky anachronism. Over the years we've developed methods to compensate for this, from advanced moderation/administration strategies to utilizing various clever plug-ins, and the functionality we've come to expect from a forum package basically limits us to expensive custom builds or high-end commercial solutions. But still, they suck.
Unfortunately there's nothing better right now, and forums are still the backbone of pretty much any large online community. How else do you provide a method for hundreds, thousands, or tens of thousands of people to get together and enjoy structured discourse?