here's some commentary I found which may shed some light:

The requiem was embraced by a whole continent. The general opinion was in favor of the requiem. Many shared Brahms' opinion that -"only a genius could have written such a work". There were, however, those who were less enthusiastic.
Hans von bulow, the great conductor, called it:"An opera in ecclesiastical robes", and Wagner, having heard the requiem, is reported to have said, simply, - "it is better to say nothing.."
It is true that the requiem has something of the operatic in it. However, in good performances, the dramatic touch only strengthens the power of this composition.
The question of whether it is truly 'ecclesiastical' is more problematic.