I've always labored under the understanding that a higher angle is a steeper angle, and therefore more "aggressive."

I understand that steering with a more aggressive angle will be more precise and more "twitchy."

What are the other characteristics of shallower or steeper angles?

For example- my 1984 Schwinn Voyageur SP is stated to have a head tube angle of 72 but a seat tube angle of 75 (which seems really steep to me- even compared to most "race" bikes). I've always had a hard time getting any other saddle to work on that bike, other than what came with it. I eventually swapped out the seat post for a more adjustable Superbe Pro. I'm not sure what else that 75 STA does for the ride of the bike, or my pedal stroke other than making it difficult to find a saddle/post combo that works.

For the record, the seat angles on my Miyata 1000 is 72, my 620 is 73.5 and my 720 is 73.