For the discussions regarding the negative aspects of a like button, are they really referring to the negative aspects of the feature, or are they comparing it with forums that have poor etiquette and poor moderation?
Call me cynical, but etiquette on the internet is a result of moderation. The forum I mentioned with reputation points is a well moderated forum. It just has a different dynamic, not bad, but different. That could be do to the reputation points, or just the subject and group it draws.
If Peter's idea stops people from typing + 1, then give it a go.
The same people who + 1 regularly add nothing to the forum apart from wasting the time of those reading it. If they want to up their post count type something of meaning.
While I strongly disagree that +1 is just a "post count incrementer", IMO the logic of people posting just for "like count" applies here also. Even if there is no "official" tracking of like counts, people will start to aim their posts to see how many likes they can generate. Frankly, I think +1's can be really useful, where "likes" become the self gratification contest.
On the truck forum that I used as my example above for reputation points, I posted a question about a problem I was having. I got 10 responses, or so. 5 of the responses were +1 to check a relay in the fuse box. Quick and easy, that was my problem. But, now I have to go back and "rep-up" everyone that responded, regardless of the answer, or I'm the jerk who asks a question and doesn't rep. Even though I posted a response thanking everyone for the answers and stating that it was the relay. +1's = good/useful, reps = ugh.
I really don't think likes will be a horrible thing. Maybe it's a quick thing to turn on in the forum software. If that's the case, no biggie. But, if it is a bunch of work for the forum admin, I don't see it as being that great of a feature.