There are a couple of differences with Seattle:
1) Antonio Daniels is FINALLY playing like he did in SA long ago and living up to his promise. He has tried to be the scorer in the past rather than allowing the game to come to him and be the orchestrator. Barry's leaving was good for Antonio because it has made him "the man." Daniels has always been a fairly good defensive player and now he is beginning to gain confidence in his game -- a very good thing for the Sonics. But Daniels isn't the main reason that Seattle is doing so well.
2) Ray Allen is playing like he is out-of-his-mind. Ray has always been a great shooter, a decent scorer and an underrated defender but as good as Ray is playing, he isn't the reason they are doing so well, IMO.
3) Rashard Lewis is also playing very well -- and isn't feeling the pressure like he was at the end of last year of being the #2 option because of the play of others. That's a good thing for Rashard who will have times where he is the #2 man but times where he just disappears.
4) The BIGGEST reason for Seattle's upside is because of the addition of Danny Fortson who gives Seattle what they have been missing for YEARS! A tough guy in the middle who will play enforcer. And stats don't tell the whole story for Fortson's impact -- the centers for Seattle (Jerome James, etc.) have been playing soft for years -- Fortson is taking it to them in practice and toughening them up. He is also leading the charge and setting the standard for rebounding and post defense. He isn't starting but he is helping add a toughness to Seattle that they've been missing for years!
However, Seattle is catching a lot of teams unaware -- they won't continue to do that. I think they have a legit shot at the playoffs but other teams are going to be much better and Seattle is playing at the top of their potential. Houston is going (okay, should) get better, same for Minnesota, SacTown is already improving, Dallas is already there, SA is already there, the Lakers are going to make the playoffs and I think Utah will as well -- so that leaves Seattle and Denver fighting for that 8th spot. The Western Conference is going to be exciting this year -- it's going to be exciting to see who makes it into the playoffs because the West is deeper than ever.