This is one of those episodes that people still talk about, for both good and bad reasons. It's the perfect example of something that was daring for its time but now looks quite dated.
To today's audience, it's hard to see what the fuss was about. *This* won an award from GLAAD? The simple explanation is that the 80s were a lot more homophobic than people remember. Many of the strides made by LGBT people in the 70s were met with a backlash during the Reagan era. By 1982 AIDS was a major topic of conversation, and to many gay people it felt like they were being shoved back in the closet.
When Sam's old teammate Tom Kenderson comes to town to promote his memoir, Sam is excited to see his buddy. Of course he hasn't actually read the book, and Tom's coming out throws him for a loop. Once again, the reactions are all true to the characters. Coach doesn't judge Tom at all; Carla (who lusted after the former catcher) is disappointed; Diane is most concerned with Sam's reaction. Sam has a moment of doubt but decides to stand up for Tom. Surprisingly, it's Norm, Cliff, and the rest of the barflies who are most upset. The stage is set for a battle, and Sam must choose between bleeding heart Diane and his bigoted customers.
One of the complaints from critics watching this now is that the writers took the easy way out by making Tom an outside character who is never seen again after this episode. That's valid, but it was probably expecting too much for a new show to feature a main gay character in 1993.
The other criticism is that Cliff and especially Norm are the leaders of the homophobic mob. You could argue, however, that making the anti-gay bullies also outsiders to the bar would be a cop out. As well, it leads to one of the most famous (and funny) last lines in the whole series.