We went through a period when every slow white PG was considered as "another" Nash, which was absured. Steve's amazing vision and ability to pass in traffic but also outside shoot has been all but impossible to replace. Sadly, his inability to defend even moderately quick opponents has been a huge problem ever since he joined the team. It is a frustrating contrast.
It is interesting to note the Mavs had a good defense in the 2002-03 season when opponents shot only 43.9% (34.0% for three) and the Mavs had a 6.8 ppg scoring advantage over opponents. His weakness was offset by having Shanw Bradley (a 7'6" shot blocker behind him) as well as LaFrentz and Dirk. By contrast, with the Suns there were only weak side shot blockers while here.
IMHO, the Suns Front Office has never really accepted the need to have guys with the skills to offset the weaknesses of Nash and Amare. Lopez might turn out to be the shot blocker they need, but that only adds to mismatch Hedo may present. But in the next few years, there is hope that Dragic will evolve into a top offensive PG, so the current defensive problems will not be so obvious.