I started commuting on an old 35+ lb mountainbike with knobbies.
Then I got a 14 speed old steel frictionshifting road bike that fit me well.
Now I'm going somewhere inbetween with an Aurora + rack, fenders, Arkel Bug, etc.
I'm still learning about this, and for example, the difference between even 20 psi (90 to 110) or some skinnier tires, or just getting into the habbit of cleaning and lubing my drivetrain every week seems to make a noticable difference to me.
I keep looking around and seeing people pitching low resistance bearings, lighter wheelsets, supra dupra go fast bits etc.
Now, I'm not going to spend too much on loosing weight off the bike, as if it's that important to me, I'd just put less than the 20lbs of stuff that goes into my backpack currently, but for example bearings, do these type of things make a really noticable difference to an average commuter?