Charles Atkins of Dyvers fame posted over on G+ about other people's posts. Do you chime in, or wait a while so you don't seem like you're piggy-backing?
Me, I'm 100% pro-post here.
An idea shared is an idea multiplied.
If I write something that triggers your own thoughts, throw them in the comments.
Even better, blog about it on your own blog and comment.
Either way, if you write a related post, don't be shy about telling me and everyone else. Put a link in the comments (and HTMLize it if you can.) If you take an idea I have and run with it, people who like most post benefit more if they can easily find your post.
By all means expand on the idea. Explain how you'd do it, if it's different. Take the idea and go new and interesting places with it. Link back. The third w in WWW is the "web" part, so connect up. As long as it's not spam, I'm not going to clip the comment.
Posts like the early GURPS 101 posts, Melee Academy, and even concepts like GURPSday (started by Doug) all celebrate multiple writers spinning around the same ideas. Lots of times the circle of GURPS-centered blogs have deliberately coordinated and even more often just randomly sprang off of each other's posts.
So if you have more to say and you can take Post X and make it Post X+1, do it.