My response in class, was that it really depended on the person. Some folks walk into a "normal" temperature room and are already perspiring. I knew a fellow that used to sweat profusely after just 2 sun salutations. Personally, it's very rare that I sweat, but I can even run a few blocks without any dripping.
So, in the end, sweating or not does not determine if a practice is working for you or benefiting you. The real question is,are you over- efforting? Are you putting forth so much effort, that you forget your breath or worse yet are holding it unconsciously? If your breath is steady, full, and even, then go for it.
The breath, not the sweat, is the most wonderful sign post to tell you if you've strayed from the path of beneficial or not.
There have been a number of folks posting about the breath lately. To read more about it, check out Just Breath & Yogademia.