i agree, Martin Hannet was very much essential to the success of Joy Division. where the band was messy and umm... ametuerish in stage, Martin hones their strenghts and strengthens their weaknesses. there's no doubt that Peter hook is an excellent bassist, and Stephen Morris is a great drummer, and both elements are emphasized on "Unknown Pleasures." he also introduces the synth to the band, not as a main instrument but as something that can successfully add another element to their sound. However, if you listen to a lot of Martin Hannet's production, you can tell he follows a basic formula; the way he mixes the drums. they all have that sparse, slightly echoed reverb to it. Happy Monday's "Bummed" has it, and so does the Hannet produced version of Orchestral Manoeuvres in the Dark's "Electricity," which I maintain is the best version of the song, no matter how unhappy Andy McCluskey and Paul Humphreys may feel with the end result.
Perhaps New Order wanted to start something new after 1981. I don't think Martin would've been very happy with the harsh drum sounds of "Temptation" and "Hurt." that said, "Everything's Gone Green" and "Procession" has some of the best production i've heard, and "Movement" remains one of the cleanest and warmest sounding albums in New Order's catalog.
cheers.