Quite an interesting thread indeed! So maybe it is worth revamping it
I cannot but wonder if you are focusing too much on slave labor? Slave labor was present virtually everwhere throughout Ancient era, Middle Age (not only in that lesser form called serfdom) and Modern times, and partly even in the contemporary epoch, yet the levels of technological advance of each polity were greatly different. Slavery might well have played a role (even a major one, as far as labor-saving devices are concerned; the example drawn from the Life of Vespasian is quite intriguing in this respect, showing an actual conflict between economic and political considerations: could you please specify the source of that quotation?), and it would be definitely worth carrying out an in-depth comparative study on the (both diacronic and syncronic) relationship between availability of slave labor and tecnological advancement; still, even if such an inverse proportionality were to be conclusively demonstrated, I do not think this might be claimed to be a decisive point in a discussion basically comparing the tecnological advances of Hellenistic or pre-Roman Empire era and the Roman Imperial one. Most likely the truth lies (unintended pun) elsewhere...
As to the mentality argument, the most often invoked one (and often the most difficult to define and thus advocate/challenge), I hold that this is the key factor explaining the relative backwardness and above all stagnation (from a technology standpoint) of Roman Empire society and economy. If a conservative mentality (and thus social order) does not constitute in itself a deterrent to creating technological innovation, it is definitely a powerful deterrent to the spread and uptake of it.
Another interesting aspect - oddly neglected in this otherwise exceedingly rich thread - is the role of military-driven tecnology innovation (just think of how many military spinoffs have lately become standard features or even staple fixtures of everyday life such as the Internet, computers, radar, GPS or even microwave oven; coming back to Romans, their military and tecnology were strictly intertwined: just think of their engineer-soldiers able to build and repair palisades, castra and war machines as well as to trace and pave roads). Of course tecnology alone is - and was - not enough without organization, discipline, tactics and strategical vision (and Hellenism was no exception to that: just think of Archimede's "burning mirrors" and shiplifting machines that did not prevent his sieged Syracuse to be seized), but Romans, being practical and pragmatic, had plenty of them. My point is that, at or around the apex of their expansion and power, Romans had basically no major incentive or need (read: external enemies) demanding or justifying large investments in military tecnology (China, the only potential antagonist envisageable, was too distant for that time's geopolitical horizon - the core of the Roman world was always to remain the Mediterranean basin - and no relationship between those two empires has ever been documented or otherwise demonstrated to date)... Therefore it would be interesting to know more about the innovations in weaponry (if any) used by Romans against their most formidable enemies like, e.g., Parthians (whose empire, though, was in fact shrinking in the period at issue, having attained its climax around 60 BC)...
