'Unlike his predecessor Bruckner, Mahler expanded the percussion section with new instruments and new uses for old ones.'
Be careful when comparing Mahler and Bruckner, as although there are some similarities in output, the music and philosophy behind the two men are very different. In fact the only definite common feature the two composers have is the expansion of the symphony's form to enormous proportions. Mahler was deliberately 'pushing the envelope' of what the orchestra was for in order to present overwhelming visions in which the whole of creation and experience is crammed into the score, often with frantic and sudden changes of direction and a sense of a very human perspective. Bruckner, on the other hand, was entirely disinterested in innovation for its own sake; he wrote music concerned with the gradual and logical unfolding of material - no less overwhelming, but instead shaped into a contemplated and deeply religious experience which tries to transcend human experience. I am not aware of the extent to which Mahler was influenced by or even interested in Bruckner's music, although it is likely he would have been aware of it by the time he was an established composer.
On the same track, the points about orchestration are interesting - Bruckner's almost subordinates the individual colours of each instrument into a whole much as an organist mixes stops (indeed much has been written on the organ-like sound of his music) whereas Mahler heightens the individuality of instruments. Interestingly, both composers have a comparable level of counterpoint in their music, however Mahler's is the more colouristically striking for this reason.
A final point has not been mentioned yet - Mahler's ability as a conductor (which he actually considered his primary occupation). Aside from raising orchestral standards it would have been difficult for anyone to conceive of the orchestra as he did without having spent the amount of time he did on the podium.