Some good info here.
Well, it's more than just "great post" Actually, if you have integrated Google Plus comments into your blogs you can see just the +1's and "great post" are just a way to get themselves attention as well. I've used both on my blogs to A/B test comments. I've noticed that some use +1's to get themselves attention as well.
My recent post How To Gain Leadership Inspiration From A Person vs. A Title @DebbieLaskeyMBA #bealeader