I know the Giant frames have changed significantly since '06, which is when I got my Trance. Their STP however has stayed pretty much the same.
The Glory which now falls in line with the rest of the range seems to have significant weight savings since last year, I really like the new look.
The bike industry is just so progressive at the moment, not many brands out there that aren't changing at least something, which I means most bikes wouldn't really hold their resale hugely so I wouldn't call one an "investment". However on the other hand you have a more affordable second hand market which is a good thing for those that can't afford new.
Yeah it sucks if you have to keep buying new bikes just to keep up with new technology though, but such it is for those who need to have the latest and greatest. Weight is just one of the parts of the game I guess, a 400g drop wouldn't surprise me, paint certainly does weigh more than one might think, and well the other 200g wouldn't be too hard to drop, particularly in spec.