I have never rode an 03-04 YZ450. Like you, I do have an 06 YZ450, and I think it's very strong compared to a friends 07.
i have a 03 yam, and man that thing pulls your arms off. (especially for me weighing in at 130)
and i agree with the hydrulic clutch statement those things feel so smooth.
and in this months dirt rider it says the 2010 yz450f will have the engine tilted backwards, efi, really weird exhaust
, air box where gas tank normally is, and gas tank under the seat. they apparentley tilted the engine back to get the light front end feeling of a two stroke. i think they compromised that light feeling with an estart though.