Worked on a product for a while with basically the same value prop. User acquisition is tough when you're essentially competing with a vast array of topic-specific forums scattered across the Web. Many of these have very large existing user bases.
In the end, things didn't work out out. I think part of the reason is that people only care deeply about a small number of topics and they're willing to go where the community is.
As software developers, We've been taught that a good solution to a problem is a "generic" one, a "one fits all" solution.
for example, in this case, a solution that would fit "all" people interested in "any" topic.
The thing is, for all non-tech problems, the best solution is usually the specific one, the custom made one.
Most of the time when you try a generic solution, competitors specializing in a specific solution would beat you.
Jeff Atwood wrote about a similar idea and called it rule of the 3 [1]. I vaguely remember that he also wrote that they used this rule when starting StackOverflow.
In the end, things didn't work out out. I think part of the reason is that people only care deeply about a small number of topics and they're willing to go where the community is.