LDC. if I may abbreviate the name above as such, I believe put it really well.
(Hmm, any relation to Wesley Crusher?)
One thing I might add is: Unless it's a brand new request thread or very fresh plot post, do consider that people sometimes have their interests wax and wane or their mood about doing specific plots that moment can change. Sometimes they just don't get around to updating a "status" for each of their plot ideas. Or they might forget some (or even a lot) of the details that were suggested, or what else they imagined about a plot idea but didn't write out at the time. Pointing to the idea with direct quotes and very similar words can help, but it's not always guaranteed that the idea is still ripe or all the same.
So, I think that while it can often be good to show that you've actually thought about some ideas in their threads, and yes you might also find them happy to pick up a particular thing they posted earlier... It's good to talk about what appeals to you about their tastes more generally if you can too. It might feel a bit of a leap or testing the waters at times (and it's okay to say so too), but why do you think/hope they might be a good match with your ways and likes?
Personally at least, I also appreciate it when replies come with at least some quick previews or at least, kinda evocative hints of how they would like to add to a/any plot (or if you like, "fill in parts") and maybe even specifically "fan the flames" to keep things lively. Particularly imagining some power imbalance, I'm often wondering so what is
their pace and focus going to feel like when their character(s) have all this initiative. But I think lots of people appreciate that kind of thing. Some people always request writing samples; I just like to see it flow into the more exciting replies.