Flavour in different cuts I read somewhere that different cuts of beef have different flavour intensities. The general trend was that the more the muscle was used (ie: rump), the more flavour it had but the tougher it was and the less the muscle was used (ie: fillet), the more tender but less flavour it had.

So it seems like flavour and tender are inversely propotional to each other.

Is there a basis behind this? Whilst we are at it, what is it that makes a cut 'tougher' than other cuts? Is it that the meat is more 'dense'?
I don't believe they are inversely proportional.  You can a tender cut that has a lot of flavor, such as a good rib-eye steak.

For flavor, the general rule of thumb is "fat is flavor".  Were not talking about all the thick and hard gristle fat on the edges, but the marbled fat speckled in the meat.  That fat helps provide flavor and moisture as it breaks down.

For toughness, it relates to where on the animal the cut is taken from.  Meat taken from areas where there is alot of muscle activity and movement is going to be "tougher" and required the more low and slow cooking approach to break everything down.  Meat taken from the opposite side of the spectrum that gets very little physical activity is going to be softer and more tender.

At the end of the day, you have to cook the cut of meat properly.  You can make a tender cut tough and a juicy cut dry if you don't cook it right.