There are, I think, two key requirements for conducting Mahler well. On the one hand, the extreme dynamic contrasts in his symphonies and the raw energy of his crescendoes call for a certain power. You need be able to hold the orchestra in check during the lesser explosions, so that they have a little extra oomph to give for the larger ones. (It almost goes without saying that you need to have the courage to abandon all restraint at such moments, and to inspire the orchestra with that courage). Managing this rise and fall of energy requires a sense of the big picture.
On the other hand, everything in a Mahler symphony that isn't a violent outburst, which therefore means about ninety-five percent of the music, requires the utmost local control. His entrances and exits are subtle, his orchestrations finely-calibrated, his interplay between sections of the orchestra is remarkably complex. A poorly-conducted Mahler work turns into a muddle: the parts collapse into each other and his phrases lose their shape. To get a Mahler symphony to sound right, you need to watch out for all sorts of tiny details.
Gerard Schwarz is a good Mahler conductor, but not a great one. That said, I admire him for the manner of his failure to be great, because he has the detail work down perfectly. Any fool trained in conducting can conduct a Mahler symphony loudly, but it takes real skill to conduct one cleanly. I saw Schwarz conduct the Third last year, during which the final movement was hardly as gripping as it could have been, but in which he brought the difficult first movement to compelling life. His version of the Sixth was similar: the final movement didn't have the overwhelming emotional impact I know it can have, but he did brilliantly by the first three movements, teasing some wonderful ensemble playing out of the orchestra. And as for the Kindertotenlieder which opened the program, their elegant use of a restrained palette fit his style excellently.
And as for that lack of power? It wasn't such a big deal. Benaroya Hall intrinsically compensates for such things.