Well, that's Jack for ya. He's a known hothead and his own worst enemy.
I'm a long time Jack Bruce fanatic. His playing on Crossroads (Wheels of Fire, live), Badge, White Room (live) is way, way up there.
Nonetheless, he's nuts to disparage Zeppelin like that. Cream was only around for 2 or so years and though they broke a hell of a lot of ground, they didn't branch out too much. Zeppelin was a far more diverse and textured band. Compositionally, Zeppelin wrecks Cream. If Jacks going to be critical, how many times is he going to milk that White Room / Tales Of Brave Ulysses chord progression? I swear he's used it in at least 3 of his post-Cream albums.
On so many levels, Zeppelin is simply in another league from Cream:
Exhibit A: John Paul Jones and Bonham injecting the funk into (what was then) heavy rock - brilliant.
B: Jimmy Page was a far more diverse guitarist with far more on the ball compositionally and chord-wise than Clapton. I'm sure Clapton would be the first to admit this. Clapton took the blues to probably the highest level of refinement with Cream but he was/is very bound and restricted to that and Jimmy Page was a damn, damn great blues/rock guitarist in his own right (like maybe the best) and far more diverse and dynamic.
C: John Paul Jones is a more diverse and (in my opinion) accomplished bassist than Jack. Again, Zeppelin branched out and accomplished so much more than Cream.
D: Zeppelin had two very on-the-ball composers: Page and Jones. Those guys know their music and were amazingly creative and prolific for a good long run.
E. Zeppelin was a more subtle band than Cream.
I am a HUGE and long-time Cream FANATIC but I can go on and on. Jack was spouting off, being Jack. His personality (and the fact that Ginger could hardly stand it) is why Cream didn't last.
So what if Zeppelin down-tuned for their reunion? Can anybody expect Robert Plant pushing age 60 to sing those incredibly high vocal parts he did in his 20's?