The bike is getting heavier and heavier... Will post my packing list soon for feedback.
The weight only matters if you're on a schedule.... That said, there are a lot of mechanical and esthetic advantages to a minimal load.
Unfortunately, if you leave it up to 100 people to tell you what they consider to be the most essential item they bring, you'll likely get 110 absolutely essential items.
As you read through the comments on your list, remember to (a) don't take offense and (b) heed only those recommendations that truly strike a chord for you.
Matthew Lee liked to point out that on 2 week races, he only took a few items in addition to his everyday carry of tools and parts. It's surprising how little it takes to sleep, eat, and keep moving.
For comparison, I ran into a fellow time-trialing the Great Divide Mountan Bike Route on a rig that weighed ~35 pounds fully loaded including mud and water. On the other hand I had a great time taking 4 times longer to complete the trip on a bike and racks that had a mass of 34 pounds and went ~80 pounds fully loaded..., including spare pedals, an electric shaver and computer, but on the other hand, without a stove.
That make sense! I'm not in rush. I plan to use 3 month to finish the trip. If I bike 50 miles/day on average I can finish in 78 days, with 12 rest days. I will bring cameras and a laptop with me to do some video editing along the way so it's probably gonna be a heavy load.
I'm mostly unsure about clothing. Do these clothes make sense for northern tier (East to West) route June - September?
Two cycling shirts
A pair of cycling shorts
A pair of cycling pants
A lightweight dawn jacket (Uniqlo Ultra Light Down Parka)
A thin jacket (Patagonia R1 techface jacket)
A rain jacket (Rei Co-op Men's Junction Cycling Rain Jacket)
An ultra thin jacket
A short sleeve tshirt
A long sleeve tshirt
A pair of thin sweat pants
A pair of thick sweat pants
A pair of jeans
A pair of shorts
Two low neck socks
Two long neck socks
Two pairs of underwear
A pair of cycling shoes
A pair of sandals
A baseball hat
A gaiter
A pair of UV protection sleeves