Sig restrictions in Classic?   

But we were told the only change was a simple variable tweak of max_block_size, from 1MB to 2MB.   

Sig restrictions sounds like something that needs more than "not much" testing.  Such a change is likely to become ossified and semi-permanent, so we have to get it right the first time.  I hope Toomin is (relatively) sober when he tests the sig restrictions!   

What else is Classic hiding and/or downplaying?  More XT-style Easter eggs, like Hearn's checkpoints and blacklists?