I'm in favor of "like" to eliminate +1 posts.
I'm in favor of "helpful" to help me find useful posts in long threads. Really long threads / interesting topics get a wiki to help distill information but not all of them. I'd like to be able view a thread with posts sorted by most helpful first (yes I know that could make understanding chronological history of a long active topic difficult).
I don't need to see who likes or marks a post as helpful.
Anything can be gamed and if these features aren't useful they can be ignored or turned off later. I see little downside in trying this if the forum code supports it.