Here is the defining element based on my reading over 20 pages of different ATP/WTA threads.....
For someone who is in favor of ATP forehand.
ATP forehand is anything which looks elegant and stylish. Also anything the top10 up and coming players using. Also anyone who makes it look like unbelievable speed on TV. The usual argument is that ATP-style is better since it is what almost ALL up and coming players are using (according to them).
For someone who is in favor of WTA forehand.
Any woman player who hits the ball hard and flat. Any guys who has relatively larger swing, and better be in top50. Usual argument is that, if X can get to top X with WTA-style, that is all I need, and looks easier.
Other than thse.... no other defining elements once you dig deep into the arguments.
Especially most top10 women, people disagree whether ATP or WTA... for example Iga/Sabalinka/Amanda... ATP crowd will say more closer to WTA and will push tons of videos.... WTA crowd will infact push the same vidoes and claim more closer to WTA