I run a small private forum, which was (until now) running on phpBB, one of the most used software for forums.
There were some problems with that.
Let's begin with the plugin system.
What they call plugins is basically just a glorified search-and-replace mechanism which instantly fails if two plugins try to edit the same file. For me, this resulted in loud howls and some downtime as I tried to install a Thanks Mod.
Another fun thing is the UI in general. Once you have found your favourite thread to read (you had to click through about 4 buttons just to get logged in, then tried to use the search, which unfortunately doesn't search post titles or usernames) and now want to read the next page, you spend about 20 seconds just finding the next page option and then some more time clicking on this 4-pixel button, you tell yourself why pages? weren't computers invented so I don't have to turn pages anymore?. You gloss over that and want to insert your thoughts into the discussion. Pressing reply loads a new page with a very small input box, which cannot be extended in size. Why is it on another page if it's that small? Why does it reload the page when I request a preview? Why do I have to click on next after publishing my post?
All in all, phpBB just doesn't feel like the twenty-first century. It more or less feels like a military submarine built for the World War I, but with a fresh layer of paint from 1950.
After I have put about a trillion hours into trying to deal with stuff like likes, youtube/soundcloud embeds and other such trickery, I finally gave up.
No! I found new hope! Discourse is the thing I have been looking for. It's clean, it's actively developed, it can moderate itself, it has notifications, it automatically converts any link you post into some useful representation of that link, and for the love of god, it is not built on a language that behaves like thinking putty (php is soft and easy to shape, but once you let it sit for a few minutes it flows back into a pile of mush; also, it falls apart when you try to make large structures).
To read more on the philosophy of Discourse, here is a blog post from the founder, Jeff Atwood.