Mavs definitely need to improve on defense to be a contender, but good team defense takes a while to build. Half of Dallas' rotation wasn't here last year, and even though some players like Tyson and Barea have been Mavs before, it's still going to take time to create that defensive chemistry.
It sure would help to get a better starting PG than Jameer Nelson though, especially on defense. If only Jason Kidd was a few years younger and still in the league he'd be the perfect point guard for this team.