- is there still a chance to compete with established Forums ?
Absolutely. Many big sites lose touch with their users, and unless you are giving them things that keep them their, then there is always room to change.
- does it make business-sense to operate a Forum ?
Running a forum for money is completely different. You don't make money from contextual advertising... not vs. paid advertisement. You need to understand and know marketing to really leverage the financial profitability of an online site. One way to make indirectly from contextual advertising, is that you use it and then sell the ability to upgrade the users account for a low annual fee, ie. $10 to remove ads. You can do this for a lot of forum aspects.
- what is the best niche to be in, to operate a profitable Forum ?
As stated above, if your not passionate about what you do, then its useless doing it. You will often make the most money from something you are passionate about.
- is it possible to make a living off from running a Forum ?
It can be done, though you need to establish the user and post base before you will see any real profit.
- what are the key-ingredients ? Is it just "passion" or are there any "secrets" ?
I would say... passionate about the topic + keen marketing sense. The ability to leverage an idea for a worth is what delivers money.
Example. I run non-profit sites, though do ask for donations to aid in the annual running costs, as they are several thousand per annum. Putting a donation link on your site is pretty useless, because people look at it and think others may donate. So you set a limit and use something like ranks to isolate who has donated, who has not. The small inclusion of an image beside a users name on every post saying they have made a donation representing a worth changes every others users opinion on that site about making a donation. Some will simply not want to be different and donate to get the icon, the recognition so they don't feel different. Using different levels, you will get others who then compete in donations total to obtain the highest icon they can get, so they stand out and feel more important than another. Think auction strategy... to leverage $$$ to play humans against one another... marketing 101.
Think of it like reputation or post count. People really do strive to compete with those free things. Understanding human psychology and behaviours is very much encompassed within marketing itself... with such understanding you will be surprised just how much money you can leverage over time if you put your mind to it. The big supermarket chains and such use it constantly. From the colour of something to grab the attention of the eye to the position or location, having to walk around something vs. past it, etc. All good marketing. There are billions online trying to make $$$... the majority failing. Do it smart and you will succeed.