I think we should turn on kudos for comments which are made on Feature Suggestions.
I believe the main downside would be that we want to keep the focus on the kudos button on the original post, since it is the "vote" button for that idea.
However, I think most users would know that the vote must be made on the original post. Additionally more direction could be given through verbal and graphical cues.
The plus-side of adding kudos to comments is that it makes the discussion more interactive, and it allows a method of value-voting the comments you agree with. If a comment received several kudos, it would draw the attention of new visitors who would realize its a popular comment.
The comments area is a discussion regarding the merits of a particular idea. This also helps create a way to have more balance in your voting system. Here's why. Currently, the only vote is a "yes" vote. That's fine for gauging user interest, but is limited in value without another option.
Suppose someone posts a comment with compelling reasons why an idea SHOULDN'T be implemented? Allowing kudos on that comment creates a more balanced snapshot of opinions, where you can then gauge the opposing interest, and you're doing it without adding any sort of "negative" or discouragement to the main voting system.
Additionally, if someone makes a great comment, why not reward it with kudos?
For example, see the comment I made on this post, after the original post included no text other than the title:
I could have gone and made my own new Suggestion post, but that is discouraged because we don't want duplication of ideas. So adding kudos to the comments allows those comments to accumulate merit as supporting or opposing arguments.