Greatness (IMO) is defined by the big wins in the biggest races against the best riders. The wins in smaller races are wins, true, but I feel a bit uneasy when those wins are used to make up for the lack of 'big' wins.
I find it difficult to call a rider 'great' if they haven't won either the Tour or the Worlds RR. Of course there are some exceptions - Kelly, De Vlaeminck, Cancellara. But there's only a handful of them.