Cassel was a complete failure and he arrived at the end of the season, along with PJ. The only real free agents they signed were Posey and House. Guys like House are a dime a dozen - Posey is the only one who really made an impact. And I agree we need somebody like him if we are to win the conference this season. But to make the playoffs? Come on ...
As far as other teams improving, not sure I agree about any of your examples. Chicago will be worse next season, since Rose will surely not be as good as hinrich in his first season. Not even Paul was great in his rookie season (he was great as a rookie but he wasn't a top PG in year 1).
Milwaukee got better by a small margin imo. They had a 28 or so wins - do you honestly think that a guy like Jefferson can get them 15 more wins next season?
Atlanta didn't make any move yet, so there's no reason to think they will be anything different than they were last season. They will probably be a .500 team again. Same about Philadelphia - they actually overachieved last season and I expect them to drop back a bit.
And internal improvement applies to all teams ... Yes, Howard will be better, but so will Bosh, Calderon, Bargnani
and Moon. It's extremely rare for a team to make a big jump through internal improvement