When you add more topics, I would pay for API access to get an hourly list of popular stories in each topic. If this is ever added please let me know :-)
We're thinking about doing at least daily digests – with the top x stories of the last 24 hours – for each topic, via email and/or RSS. Thanks for your suggestion!
If you offer RSS I wouldn't have to pay (I assume) - if you had an API I would use it for commercial use.