I don't understand the "build quality" comment either.
Here is my opinion, with a bit of background so you know where I am coming from:
I've always owned and preferred plastic Mac notebooks. I just like the look and feel better. My last computer was a non-unibody plastic Macbook. After it had numerous logic board and other failures, Apple replaced it and gave me a unibody plastic Macbook.
I liked it, BUT they had taken away several features that I liked from my older Macbook, namely:
1) Firewire
2) External battery monitor
3) Infrared port (for remote control)
I went back and forth about what to do, and compared the Macbook and the MBP over and over (this was in November 2010). Finally I decided to change over to the Macbook Pro. Even when they had identical processors (late 2010) I just couldn't justify the Macbook to myself given the differences in features for $200. Now, with the MBP having added a better processor, a better camera, and Thunderbolt, I can't imagine buying a Macbook considering the price difference (yet I love the plastic and don't like metal). I think that now the Macbook should be more like $799.
But okay, back to the comparison. Here is what you will get on a Macbook Pro now that is different than the Macbook. It seems very very favorable to me, for $200 (the price difference with the base computer, not options). I'm not counting the case as I'll mark that up to personal preference. The first four items are new as of 2011; the others existed in 2010 and still exist now.
1) Considerably faster/better processor
2) FaceTime HD camera (vs. iSight non HD camera)
3) Thunderbolt port
4) Larger hard drive (sorry, forget by how much... maybe about 1/3 larger?)
5) Firewire 800 port
6) External battery monitor
7) Infrared port (for remote control - the control is ~$20 sold separately)
8) SD card slot
9) Backlit keyboard (I had always wanted this

)
10) Subwoofers for the speakers
11) 4 gigs of RAM vs. 2 gigs of RAM*
*MBP is upgradable to 8 gigs; officially Macbook only to 4 although I have read that you can also make it take 8.
So, I bought a Macbook Pro in November (a friend was on the verge of buying a Macbook so he took mine). The way I got around my dislike of touching the metal was to put on a clear plastic "skin." Now my MBP is all shiny and does not feel cold, and as a bonus is protected from scratches.
Again, I say all this having been a long time fan of my plastic Mac notebooks. Just that now they have made it such a better value to get the metal MBP (even moreso now in 2011).
Miss Terri
PS: Oh, back to build quality. If you mean the cases, well, they each have their plusses and minuses to my mind. Beyond aesthetics:
1) Macbooks can crack (hinge area, palmrest), and potentially discolor; rubber can delaminate from bottom.
2) MBP's can dent, and anodizing can be scratched or eroded by sweat, etc.