Parcours is part of the puzzle. The riders are another one, sure, possibly a bigger one. But while the riders can make a great parcours into a terrible race, all the will to race in the world can't guarantee a great race if the parcours doesn't allow for the chances of the race to develop. Take the 2011 Worlds for example. There's also examples like Paris-Nice 2011 (and to an extent the 2012 Tour follows this example) where the TT was too long for the meagre mountain stages on offer, so while we got some good names attacking in those last two stages (Tondó, Samu), they were more or less GC-irrelevant, so their racing meant nothing. This is a key factor - we need to feel like the racing means something to invest our emotions in it. The Tour may have had stages where there was much more action than there have been in the Vuelta stages so far, but most of it was irrelevant, because the strength of Sky meant they could afford to give them a little wave when they left, and leave them to their own devices. Here, there haven't been many opportunities to create more than a few kilometres of racing, but the main protagonists - Contador, Froome, Rodríguez, Valverde - have all been on the move, meaning less action but more action that we can emotionally invest ourselves in.
Ultimately, the riders decide whether they want to make the race or not. But the parcours is an inducement. Today's racers seem, with precious few exceptions (Voeckler, Hoogerland, Txurruka, Chuzhda, Pirazzi), to need inducements to make the race exciting, and therefore we should make the parcours as appealing to attacking cycling as possible in order to best encourage them to make the race.
Ultimately, the riders decide whether they want to make the race or not. But the parcours is an inducement. Today's racers seem, with precious few exceptions (Voeckler, Hoogerland, Txurruka, Chuzhda, Pirazzi), to need inducements to make the race exciting, and therefore we should make the parcours as appealing to attacking cycling as possible in order to best encourage them to make the race.