DDR - great questions, thank you for asking them.
Why do I chose to trade in the first 30 minutes? Because there is opportunity there - I can see it in the order flow and price action.
What do I think the difference is between these two players (people who only trade the first hour and people who don't)? From watching order flow in that first hour, I get the sense that: there is more short-term, large paper volume coming into the market as compared to the second hour. What are they likely thinking? I'm not sure, but the order flow is just constant and aggressive once the market starts moving in a direction. It's almost like everyone is saying, "see the movement, hold your nose, jump in, and ride the wave." This pattern definitely seems more aggressive to the downside. Also, whoever is playing must be hitting bids / lifting offers as the price action is just too aggressive / quick. In. Out. There also tends to be a pause around 9:40-9:50 - sometimes this is a reversal, sometimes not. It is almost like paper is saying - time to exit, cut the winners and run.
I can't say anything about people who don't trade the first hour. It is tough to see what isn't there.
I am always trying to improve my personal trading (about breakeven in the ES now - slightly negative) and yesterday I was just reviewing my historical trade results by 30 minute ranges. I noticed that, comparatively, I tend to underperform against myself from 9:30-10am EST. Which raised the question - why is this? Just to give everyone some background, I don't trade anything mechanically - everything is discretionary and based off of market structure, that day's news, and order flow at key levels.
DDR - thank you for your questions - definitely something to think about this weekend!