Kane is an older, experienced guy who can still go and move respectably well, particularly for a big man. His character is really a heel, but, since he's still a 1980's-style heel (supernatural psychotic kind of guy with power over fire?), people tend to root for him, much like they do his "brother".
As to when he became face, it was probably when the six guys surrounded the ring at Raw 1000 and Undertaker came out. At the end of that segment, he was a face. They continued it by putting him now as the apparent opponent for DBD (I'm guessing Charlie Sheen's getting involved somewhere in all of this, although part of me cringes at the thought, but it would further cement Kane as a face).
All of that aside, I'm all for him continuing to work so long as he's still able and healthy. I don't want to see his activity level in the ring impact his quality of life.
wk