musicianiship--when is it bad or good ?

i have been attending concerts for the last 50 years. very often, i find it it interesting to read a music critic's "opinion" of the performance i have attended. invariably, a performer is taken to task for a poor performance or is praised for an "excellent" one.

why is one performance better than another if it is a matter of opinion ?

for example, if a pianist distorts the tempo by playing too slow or too fast, or with too much stacatto, or in general, takes liberties with the score, why is that necessarily bad ?