Old and basic.

That was attempt number one at pioneering reactions onto this website. I'll call it a mixed success:

pros:
it's immediately after the thing that is being reacted to (lets call this the subject of the reaction)
denotes a clear and legible emotional reaction
gives the person recieving the reaction a warm and fuzzy feeling (probably)

cons:
only one person can do it before it looks like you're reacting to someone elses reaction
has to be carried out before anyone else can post something you might want to react to
ambiguous? is the connection between the reaction and the subject clear and obvious?