
Well think about it like this, in a real car turn the wheel all the way left or right when stopped, then let go of the wheel and then press the excelerator. What happens? In most all cars the steering wheel will straighten itself out because of the caster built into the suspension. It does this pretty quickly - with a DFP in a race sim the motors may not be able to keep up with the speed at which the front wheels of the virtual car are trying to pull it to center. It can only go so fast. With a 900º wheel that problem becomes pretty exaggerated, much more so than with one of the older FF wheels with a third of the rotation.
Don't confuse this with the power or torque of the motors inside the wheel, it's just the speed that matters in this case. Both the DFP and G25 have plenty of power. The G25 is a lot faster though and can better 'keep up' with the forces it's trying to reproduce. That has an effect on how the car in-game feels in lot of different circumstances.
Before the G25 came out few years ago, the guy that designed it was very active over at the RaceSimCentral forums. He explained why the speed of a sim racing wheel was so important. When switching from a DFP (same basic internals as a DFGT) to the G25 and really testing them both out in all the race sims on the PC and Gran Turismo, the difference is huge. Some of you guys may laugh, but I'm telling you there's a big difference in speed between the two 900º wheels. That difference is mainly what makes the G25 so much better- not even taking into consideration the better materials and 100 times better pedals (w/ clutch pedal). Maybe I can find a link to some of those threads where the designer of the G25 was explaining all of this stuff to everyone, it really was pretty interesting. It was aslo really cool of a person like that to take so much time and effort to communicate with the Logitech 'customers'.