F.W. Franz, in the late 1980s WT article that featured his autobiography, described N.H. Knorr as having outstanding management skills. Others have observed that under Knorr, the operation of the WTS and the congregations was altered to more resemble one big business venture.
Other than that, compared with his two predecessors, Nathan Knorr was not a controversial personality - possibly why remarkably little is said about him anymore. (That was not the case, though, while he was alive).
Bill.