Personally, a text area is the only thing I need to express my gratitude. I'm doubtful about a UI that would "encourage" it. The last time I wrote on a GitHub issue, I started my message with a warm thankful line to the maintainer for his fabulous work. I meant it, and I didn't need any "reminder" to say so. It felt natural and important to me. I also put relevant informations to show my interest and the time I spent thinking about it. In the end, I believe it's more about the person posting, rather than the interface encouraging it, but I may be wrong.