Simon: Why do you use something other than the standard forum format? This format is really nice, but it seems like it would be more expensive ...
I actually found it to be the opposite. I have tried a few standard packages and the hosting costs would always have been much higher. They typically work great and are very cheap when you start out but scale is the issue. Lots of sites that use them for large forums are either revenue generating (i.e. they can throw money at the problem / it's part of their business) or they have heavily modified the code.
Same too looking at the hosted forum services - they are often based on the same technology stack so it's not surprising that the costs are passed on. They start as open source and then at some point turn it into a business, target enterprise users and expect hefty fees. I hate having hard limits where if the site is popular it bumps you up a tier and suddenly costs more. The beauty of cloud hosting is that the cost is directly related to the traffic and usage which should have some relationship to ad-revenue to cover it (and why being able to reduce the running costs helps with the economics of keeping this balance on the right side),
Also, even if you don't pay for your time to manage the hosting, it's still a "cost" in terms of energy and sometimes just a drain on your time so takes a toll. You don't realize what a drain managing database servers and web servers is until you stop doing it.
... and doesn't have "notify me of response" option.
At one point the emailing was an issue for technical reasons but I could re-instate that at some point, probably more as a "digest" of updated topics since you last visited. One of the nice new web-tech features though is push notifications so you could get alerts where something you wanted notification of was updated. I'm planning on building things like this into the moderation side of things ... i.e. you get an alert to check something out if it's been flagged by x people with enough reputation and have all the details needed and convenient buttons to take whatever action (ignore, delete, lock etc...). Again, all to make things easier with less risk of burnout while being more responsive and providing a better service.