Kimbrel is a weird one for me.
351 saves, a career ERA+ as a closer of 189, a K/9 of 14.7, K/W of 4.07.
Using those metrics, he's literally the only pitcher ever with at least 350 saves, an ERA+ of 180, a K/BB of 3.5 and a K/9 of 13 who isn't in the HOF. Heck, he's the only pitcher with those stats. Those are all truly insane numbers. But if we drop the K/9 out (since Kimbrel is I think the all-time leader in that), we get Billy Wagner and Craig Kimbrel (so just 350 SV, ERA+ 180, K/BB 3.5).
Wagner's gone from 10.5% in his first year of eligibility to 46.4% in his sixth. Obviously, that's good, but... I'm not sure he gets there. Kimbrel also had five top 10 Cy Young finishes, Wagner had two (though, damn, Wagner had exactly one season where he had an ERA over 2.85 -- even his final season in 2010 he had a 1.43 ERA in 71 appearances, the guy had a live arm at the end still).
Still, among pitchers with at least 350 saves, there are seven other pitchers not in the HOF (K-Rod, Franco, Wagner, Nathan, Paps, Reardon, Percival) -- I don't think any of them (except MAYBE Wagner -- Franco should probably be in just for his longevity, honestly) gets in.
And here's the thing: entering this season, Kimbrel's thrown about 530 innings in his whole career. Among guys who actually played in MLB and are in the HOF (and didn't play in the 1800s), he'd be dead last behind Bruce Sutter -- who threw 1042 innings.
Kimbrel gets in if people are impressed with his genuinely impressive rate stats. That's my take.