Herodatus has it spot on, if any one makes a list tht is wht everyone will tell you to make. plus daemon princes are also useful if ur opponent thinks they are target practise becuase tht means ur other units survive which are more dangerous, but in ur cause they aren't. they need meltas and lascannons as some one else has already said are rubbish thanks to cover and the fact you can't get close and fire meaning the rest of the squad is wasted.
finally you need rhino's!! footslogging across the board is going to fall even with ur failcannons. would love to see this army VS Tau, Orks and Nids