It's tough to base Harry Greb's quality off of him goofing around with 48 year old Jack O'Brien. We have to go off of the newspaper reports to really know how he fought. What we can base off of the reports about Greb are a few common things.
1. He was relentless and could fight inside. He was able to work his way inside of big, good fighters like Gunboat Smith, Gene Tunney, and Bill Miske despite their size advantages. Tunney was tremendously complimentary to Greb's ability to fire punches from unique angles setting up his ability to get into the pocket.
2. He was brutal. Greb didn't give a fuck and he intended on busting up the guy in front of him whether it was legal or not. He had 300 sanctioned fights and was knocked out twice.
3. The only way people beat him was by taking the fight to him. The two decisive losses in Greb's prime were to Tommy Gibbons and Gene Tunney, who were able to get off first and force Greb to step back and counter punch.
What does that mean about Greb's talent relative to fighters of today? I don't know, but I'd bet that Greb would get DQ'd if he went up against a good amateur.