JSONP API support
We had to implement a server side PHP proxy to do AJAX based form submissions with HubSpot. If you supported JSON-P then we could just do the submit directly to you correctly without the PHP proxy which would simplify things for us and anyone else who wants full AJAX forms on your site (the form loads via AJAX by default but submissions are still old fashioned POST calls that change the whole page).
Modern cloud companies offer JS friendly API options. HubSpot should too. You'll see a lot more enthusiastic development and support if you embrace the people who want to do amazing things with your software.
I agree with this!