I replaced all of my plumbing and now I have really low water pressure, what happened?

I replaced all of the plumbing in my house with new 3/4 inch copper. It was 1/2 before. I replaced the water heater at the same time and one sink/faucet. All of the supply lines are 3/4 which are connected to faucets by 3/8 ID supply lines, and to the washing machine and tub/shower by 1/2 inch copper. All of the valves are new ball valves to include the ones at the supply lines to the faucets. Yes, I have checked and all of the valves are fully open. Now I can hardly take a shower my pressure is so low. What happened? The pressure is not just low in the shower, it is low at all fixtures.