I think the problem with Heidfeld is, he needs someone to push him to get the most out of the package. Renault are lucky that Petrov seems to have stepped up a step, and the added pressure of knowing Kubica will return in time helps as well.
Otherwise, why was Heidfeld only a bit better than Villeneuve. Then Kubica got into the other car, did an amazing first race and hello, Nick Heidfeld started getting better as well to beat him. Same in other cases.
A dependant, good driver, but not the guy you want to lead your team. Maybe he could replace Massa at Ferrari. Alonso is certainly to push Heidfeld to new heights as well.
I kind of like the guy as a person. And I was exited when he started racing, as I was living in his home town of Moenchengladbach at the time. But he is no championship material.