The most scathing criticism of John Kerry is that he's a boring mediocrity
John Kerry's current position in the Democratic Party largely stems from the fact that he ran for President in the 2004 election cycle. If you look at the 2004 Democratic Presidential Primary, you'll see that he's probably the least controversial person who ran.
Compared to John Kerry...
- Howard Dean, Dennis Kucinich, and Carol Mosely Braun were significantly to the left of the rest of the party at the time
- Joe Libermann was significantly to the right of the rest of the party at the time
- John Edwards had very little experience as a Senator
- Dick Gephardt had already ran for President once before and couldn't win his home state
- Wesley Clark entered the race late and had poor campaigning skills
- Al Sharpton was obviously running just for the sake of getting otherwise un-earnable press coverage
It's pretty easy to be the least objectionable person when everyone else has liabilities that limit their appeal. So, he ended up getting the nomination in 2004.
Some of the "revering" you see is just self-preservation instinct
Once you win your party's nomination to be President, people treat you differently; it's hard to win a nomination, and if you can do it once you might be able to do it again. Which means you're potentially threatening to other people who would like to win the nomination in the future. Hence, people start sucking up to you and offer you high-profile jobs to keep you happy and prevent you from running.
Hence, John Kerry becomes Secretary of State in 2013, despite not being particularly more distinguished in the field of foreign relations than anyone else who could have been chosen (though at least he was the chair of the Senate Foreign Relations Committee for the 4 years prior), and that probably was enough to stop him from running in 2016.