Mike Piazza was first eligible for voting after the 2013 season, the first of the alleged PED "tainted" years. Rightly or wrongly, Piazza might be judged from the era he played in.
The case for Piazza as a Hall of Famer
Piazza has numerous awards to his credit. He was the NL Rookie of the Year in 1993 and then added twelve All Star appearances, including ten starts. He nabbed ten straight Silver Sluggers from 1993 to 2002. He never was voted MVP, but had seven top ten finishes, including two second place finishes in 1996 and 1997.
Piazza has the most home runs as a catcher, 427, all time, and is fourth all time in RBI. He has the highest slugging in history, 0.545, and the highest OPS, 0.922. And even though he played in the most offensive era ever, he is still first all time in OPS+ at 143. He also finished with a career 0.308 average, one of the trigger points for voters.
From a bWAR standpoint, he is fifth all time at 59.4, third all time at WAR7 (43.1) and JAWS has him fifth in history at 51.2. Fangraphs has him sixth all time with 63.5 WAR. Hall of Fame Monitor and Hall of Fame Standards both have him comfortably over the average HOF'er.
The case against Piazza as a Hall of Famer
As prolific as he was offensively, he was probably one of the worst defensive catchers in history. In the case of bWAR, a catcher is the most rewarded position defensively. In Piazza's career, he finished with a total of 1.0 dWAR. That is well below a league average catcher. Fangraphs is equally harsh on him. He has a career 20.7 Def rating. It really isn't fair to compare him to Ivan Rodriguez and his 315.1 Def rating, but Yogi Berra, Johnny Bench, Gary Carter and Carlton Fisk all were over 100.
Then there is the rumor about his PED use. None of the rumors were ever concrete, but with the era he played in, any rumor stuck with in BBWAA voters' minds.
Gray Ink, another Hall of Fame metric lists him at 101, with an average Hall of Famer around 144.
My two cents
As poor as Piazza was defensively, his offense definitely made up for it, and then some. I would have no problem voting him in, but I still would list a few Hall of Famers above him.
The voters were not nearly as harsh on him in that 2013 vote as he finished at 57.8%. Last year, he trended upward to 62.2%. Usually that means a player will get in eventually. However, with this year's over crowded ballot, I fear his vote total will finish below 2013. Hopefully he gets in soon, as I don't think the ballot will really get much less crowded going forward.