I had a beautiful answer all typed up, and the Internet ate it. But Devon Diver said most of what I was going to say, anyway.
My only addition would be that you should choose the things to carry by the dive you are going to do. If you are diving in a small quarry where a shout will easily carry to shore and there is no boat traffic, you don't need a whistle or a marker buoy. If you are diving in good visibility and very shallow, you may not need a light. Don't carry what you don't need, because three things are true: Everything you hang from yourself will slow you down. Everything you hang from yourself will eventually get lost, and everything you hang from yourself has the possibility of getting caught in something. That's why pocket storage, or neat stowage along straps, is preferred to things that dangle.
My only addition would be that you should choose the things to carry by the dive you are going to do. If you are diving in a small quarry where a shout will easily carry to shore and there is no boat traffic, you don't need a whistle or a marker buoy. If you are diving in good visibility and very shallow, you may not need a light. Don't carry what you don't need, because three things are true: Everything you hang from yourself will slow you down. Everything you hang from yourself will eventually get lost, and everything you hang from yourself has the possibility of getting caught in something. That's why pocket storage, or neat stowage along straps, is preferred to things that dangle.