Here is my re-sort with individual items ranked within their groups:

Most important Items:
Black Extractor Spring Insert / Crane O-ring / D-fender
Properly Staked Gas Key
Staked Castle Nut
Pressure Test (HPT) Bolt
MPI Bolt
Milspec Barrel Steel
Shot Peened Bolt
5.56 Chamber (my personal opinion is that the Wylde is also a very valid chamber, but not if it's to be chrome lined.)
Chrome Chamber & Bore

"Conditional" Items (meaning personal needs/taste/preference):
"H" Buffer (this has more positive result than an M16 carrier as long as a shrouded carrier is being used)
Milspec Dia. Receiver Extension (depends on stock fitting requirements)
1:7 Rifling Twist (depends on ammo choice)
Pressure Test (HPT) Barrel
MPI Barrel
F Front Sight Base (FSB)
M16 Bolt Carrier
M4 Feedramps

Relatively Insignificant Items
Double Shield Handguards
Milspec Dia. FCG Pins
Taper Pins at FSB
Parkerize Under FSB