I would have to say that the build quality and design is far greater on the 3G in my opinion. It doesn't FEEL like it sometimes because it's plastic, but it has more support because of the domed back, less volume loss from the metal grated speakers/mic, and more solid button mounts and feel thanks to internal design and metal button caps. Since it is plastic though, it can't be cut with a laser or be as precise as far as fitment goes, which is why certain demands of perfection by some people are just unreasonable from a manufacturing standpoint. (I've worked in a machine/plastics shop for some time in the past)
Unfortunately it sounds like you are just one of "those customers". Seriously, the issues you describe sound very suspicious. Stop putting L-shaped or extra long headphone jacks in it, don't keep it in a speaker dock or iphone dock all the time, and stop pushing the volume buttons so hard or hitting them on things. That will resolve the cracking issues. Then all you're left with are the so-called "light leak" problem. ALL 3G IPHONES DO THAT! Some more than others, but it's still only noticeable in a dark room with high brightness levels and you won't even see it if you use a case. They use a rubber gasket now so there isn't a metal rim contacting bare glass. Rubber moves and changes shape slightly... Would you rather have a hit on the ring transmit straight to the glass and crack it or have a little bit of protection against bumps and drops?
The only problems out of spec with the ones you listed were:
-dust under glass when new
&
-Wifi stopped working (which can sometimes be resolved through DFU restore, etc.)
Anyways, I'm just saying to people basing their buying decisions off of a topic on the internet where everyone goes to complain about the same thing, think it through and talk to the happy customers about the problem too. The issue is, those satisfied customers don't go into a thread about a problem to say "Hi guys, mine works great! OK bye." They far outnumber the people with constant build quality problems.
I have had 2 launch day white 16gb iphones and they still haven't had any cracks or 'major' light leak issues. I have used it in an incase slider case for quite some time now and have stopped using it in the iphone dock because one wrong push on it to turn off my alarm when I'm half asleep will probably make the dock connector crack.