Better information in the description would be good. Having to either derive a math equation for an integral or hunt for it online should be unnecessary and slows down learning.

Instruction text should provide formulas needed and test failures should state which test failed with what input data.

It doesn't make sense to return null when given a perfectly valid empty string unless this is explicitly specified in the instructions. The return result should be the same empty string.

I'd like the ability to unsubscribe from getting emails about a specific forum topic. Or if this feature does exist, make it more obvious by including it in the email sent and/or settings. Really, more granularity of notifications received would be nicer: I'd like to see comments responding to my own posts and katas, but that's about it.

I personally would like to see Python on here as well. I've been using Checkio.org to learn more Python, but I like how this site is designed a lot better.