There is always personalized setup - and any suspension setup can be made right "for you" by a good suspension tuner (e.g. Les at LT-Racing) - seat foam, bar risers, pegs, etc.
GasGas refines the bike year after year, gradually improving an already "good thing"...
For me, I see the following benefits on the newer model GasGas' bikes :
1. Marzocchi front/ohlins rear combination on suspension is a very good thing...
2. The motor is much lighter with new center case castings - removed the hump for the counter-balancer, etc. (which year did this change happen? I forget...)
3. The new sub-frame design drops a few pounds off the bike as well and improves ergonomics.
4. The '05 had a boyeson rad valve the '11 has moto tassarini v-force reed cage. - better all-around response and power. Great move.
5. The gasgas pipe on the '11 provides broad power and performs well.
6. Improved fit, finish and quality at factory.
7. Clay Stuckey and GasGas OffRoad US.
One thing I did like on the older bikes is those uber strong DID U-Rims that came on the bikes for a few years back 2001-2002 timeframe... I don't think DID makes them any longer.
jeff