I didn't even realize that this sub-forum existed before posting in "Ask A Mustachian." So now I'm bringing that post over here. To summarize:
Here's a few pictures of my basement:

The space is basically a big rectangle -- about 26 x 35.
In terms of use, I'm looking for a workout area, a seating area, a ping pong table, and a smaller card table. This is my current ideal layout, with the red lines being finished drywall:

My digging through the attic and garage indicates that the prior owners already did some work --
-Used patio paint to paint the floor
-Used masonry paint of the cinder blocks
-Started framing in a bathroom
My design plan is admittedly waffling, as I can't seem to decide on a bunch of things, and also not being able to decide on whether to do things DIY.
***
Drywall: As indicated in the picture, I am going to drywall the red lines -- the one long wall and the partition wall around the HVAC. My plan as of right now is to have a contractor friend help me frame. Then I am going to take a crack at drywalling the least visible wall (the wall behind the HVAC area), see how it looks, and then go from there.
Flooring: I initially wanted to not put any flooring down there, but Traffic Master vinyl planking has really caught my eye. It would be down there for a really, really long time (I expect traffic to be relatively minimal compared to upstairs) and it would look really sharp. My biggest question -- should I be using a sub-floor like Dricore, or just a thin layer of something?
Ceiling: My plan is to paint the ceiling a light gray. Seems like a lot of people are doing black, but that just looks too dark to me. Anybody with any experience painting these?
***
I am wondering if I am missing anything big here in terms of water/light/protecting the actual basement. My house has never had water issues (according to the disclosures), but I am extremely wary of putting money into finishing only to see it get damaged.
Like I said, I plan to DIY everything except the framing.
Any particular recommendations on where to NOT be penny wise and pound foolish?