The like system, on paper sounds like a positive thing to enable, but the problem with it is that it can be abused by members to essentially "push" reputations for people while snubbing others. In essence, it becomes more about popularity instead of substance. For example say someone releases updates to a mod, or releases a mod, many people will like this individual's posts merely for giving them content. On the same flip of a coin though, someone else could be focused on helping people out with their problems, or teaching people things in the other parts of the forum and generally being helpful to members but because they're not giving out content as the other user is, less people will "like" this individual's contributions even though in the end they are just as useful as the full-blown mod is.
Aside from the above example someone could get their friends to like all of their posts just to promote their reputation which once again would negate the benefit of the system if people are abusing it to push up their reputation via "likes". Then you also have the people who will like someone's posts and in return expect that user to like their posts, not unlike the current facebook like system and how many of it's users have professed to me to use it. It might not seem as though it would be a problem from the outside looking in, but similar to the posts equating to rankings, you from time to time will see users post excessively with the intention of bumping their post count since on many forums the number of posts a user has is used to denote their status within the community (not stating that is the case on this particular forum, but on other forums it commonly is how other users will gauge one another's "experience"). In the end the "like" system would probably have similar results.
Hopefully this clarifies why we've turned this particular accessory off within this site, in the end it boils down to how users would use it and the likelihood of abuse which would result with it turned on for general users. Since we can't really regulate the methodology of how and when it should be used without writing more rules/regulations to go along with it, it's much easier to just leave it disable at this time. Given that the "like" system is directly linked to a "Reputation" system it would be highly likely to be abused.