It's likely the URL is too long for GMail to handle.
The entire body of the e-mail is stuffed into a
mailto:. Here's what the whole URL looks like for me (over 1200 characters) for the Audio Recording proposal:
mailto:?subject=Stack Exchange Q&A site proposal: Audio Recording and Production&body=Hi!%0A%0AI'm supporting a proposal to create a new Q&A website for recording enthusiasts and professionals, audio engineers, producers, music composers and arrangers, to help with microphoning, recording techniques, mixing and mastering, and using the related hard- and software..%0A%0AIt's built on the same software as stackoverflow.com, a hugely popular site where over seven million programmers help each other with difficult programming problems. On Stack Overflow the audience votes for the best answer, so the answer you want is usually right at the top, not on page five.%0A%0AI'm hoping that a site for recording enthusiasts and professionals, audio engineers, producers, music composers and arrangers, to help with microphoning, recording techniques, mixing and mastering, and using the related hard- and software. would have the same kind of network effect and turn into an amazing resource.%0A%0AThe proposal process is going on here:%0A%0Ahttp://area51.stackexchange.com/proposals/4/audio-recording-and-production?referrer=A4RDO_KkyaE1%0A%0AIf you're interested in participating, go to that URL and click on the orange "Follow It!" button.%0A%0AThanks!
As you're doing, the only option is to cut and paste the body into each e-mail you want to send with the given referral link, instead of trying to automate it using the button (which I can confirm works if the e-mail client is Outlook Express).
I would say this is by design.