I'll add myself to the voices who think this UI is ludicrous. Introducing an annoyance for everyone, 100% of the time, in an effort to reduce the small minority of inappropriate commenting is a terrible idea.

If you're determined to do this, you don't need to make it a road-block for people who just want to add a comment.

• User clicks the "Add a comment" link.
• The comment form is opened immediately, as it used to be.
• The form includes additional form elements for the "Say thanks" and "Start a discussion" options, which users can click on if those things were what they actually wanted to do.

Will that ensure all people select the correct option? No.

Will that ensure that you aren't annoying virtually every user every single time they want to make a comment? Yes.