The differences, externally at least, between a 6-year-old machine and a 2-year-old one are pretty dramatic. The older one is very solid, connectors feel good, power button has nice response.
The newer one is just plain chintzy, almost afraid to plug anything into it for fear of breaking a connector. The door to the front ports was so cheap I just decided to break it off instead of waiting for it to go.
When recently it came time to replace the old machine I took isitreal's advice in this need a new computer [webmasterworld.com] thread. This is a nice, solid, and if I may say, a very well-made machine. With what I learned I don't think I'll ever buy an off-the-shelf machine again.
In fact, I spent some time this morning researching components so I can recycle the old machine's case. As soon as I figure out what the heck I can use it for, I'm off a'building again.