1) subscribing to the topic
Click 'notify' on lower and upper right corner.
2) notify to the OP that I was interested his topic
I think that the second is the most important, because it gives a reward to the OP.
He/She will know that there are some people interested on what he/she is saying / is going to say.
You can always send a PM.
I think that a good way to limit/stop all these "subbed" post is adding a counter ( or showing all their names at the bottom ) somewhere near/in the topic that show how many people are subscribed to it.