While I feel like I understand the counter arguments, I see this type of feature as being all-upside. It's just a question of whether or not someone with the skills wants to implement it.
I would argue that "like" feature actually encourages posting for a few reasons-- I see it it is a fine incentive for contribution.
Because our forums have, on the whole, an emphasis on post content, users are not exactly encouraged to post solely to voice appreciation of other posts. I know there have been about a million times I've read something awesome (in a thread I wouldn't luvdisc) that I wanted to express appreciation for. Do I send creepfest PM/VM messages instead? Sometimes, sure, but not everyone is willing to do that. A "like" button promotes that kind of behavior without forcing users to go out of their way.
Threads which have people quoting entire posts and then adding "I agree!" would become a thing of the past, de-cluttering threads and making more room for posts with actual content.
A "like" feature has the potential to send good messages to new users about what kinds of posts we care about. Lurking users who are scared to post can get some help by perusing posts that lots of others have liked-- sure, there would be terrible posts that get tons of "likes" which will cloud that message, and people who caustically oppose "new things" will try to abuse the feature to remove it. I still think it's a step in the right direction overall. The good outweighs the bad.
I find it pretty fascinating how many of our contributors, themselves, had difficulty finding a foothold in the community ("I was too scared to post", "had no idea how to contribute", etc) and stuff like this would only make it easier, not harder, to figure out what's going on around here.
Plus, how awesome will you feel when you spend hours writing a good post that hundreds of people take the time to "like"? Answer: pretty awesome.
Plus if it backfires, it gets removed. We tried. There is nothing to lose here.
As I said before, though, while I think this lines up 100% with something we want to be doing as a forum, it all hinges on whether or not someone on staff who knows how to do it will do it. I don't know if that's the case or not.