Ben Wallace's season may have been the most difficult assess.  Wallace's stats this season were near career lows in every category except for assists and offensive rebounding.  It did take longer than expected for the Bulls to adjust to playing with Wallace, and vice versa.  The adjustment period took longer than expected.  Wallace struggled to both the Bulls offensive and defensive systems.  Detroit's isolation system allowed Wallace to hang out on the weak side of the court in no danger of being involved in the offense.  Skiles' motion offense demands that all five players be involved in the offense, which meant that Wallace couldn't hide on offense.  It also meant that the Core had to learn that just because Wallace is open, doesn't mean he should get the ball.  On defense there was Wallace's well documented refusal to front the post.  

Wallace posted a 14.6 PER after five straight years above 17.0.  He also had .825 PW% , which was a decline from the previous year, but consistent with previous seasons where Wallace had a low FG%.  He collected 8.6 Player Wins, after earning 11 in four of the previous five seasons.  While his PER puts in the middle of the pack among centers.  Oliver's system with its emphasis on defense is much kinder.  So how much of the statistical decline was from a difficult adustment, and how much was from physical decline?  Offensively Wallace had a remarkably similar season to his last with the Pistons.  His FGA/40 min, FTA/40 min, and usage rates were nearly identical.  A rise in turnovers was offset by a career high in assists.  The big difference was a drop in Fg% from 51% to45.3%.  Why the drop?  Wallace dunked the ball alot less.  In 05/06 31% of his FGA were dunks.  With the Bulls only 13% of Wallace's FGA were dunks.  That's significant given how much Wallace struggles to even make layups.  Part of that can be chalked up to the change in offenses and teams, but Wallace also appeared to lack the lift to dunk often this season, settling for layups.  In terms of offensive efficiency, that dip in FG% is the difference between Wallace being an above-average offensive player and a league average offensive player.  I would have predicted a decline in Wallace's rebounding numbers going into the season.  Wallace was the Pistons only good rebounder among the starters during his entire tenure there, and the Bulls were already a strong defensive rebounding team.  Wallace's rebounding numbers in the end were not dramatically lower than the previous season.
Wallace's defense is an interesting case.  He had an identical defensive rating (DRtg) to his previous season.  His blks/40 min declined for the sixth consecutive year, but he still had 2 blks/g. His steals remained consistent with past performance. But, I wasn't prepared for how much Wallace struggles with on-the-ball defense, evidenced by his opponent production.  We saw all season that anyone 6'11" or taller gave him real problems.
Perhaps one of the biggest statistical arguments in favor of the Wallace signing was that Detroit was consistently better on both offense and defense with Wallace on the court, but the Bulls were better both offensively and defensively with Wallace off the court.  Although those numbers did improve towards the end of the season. Through the middle of the season Wallace had the worst on court/off court ratio in the league.

What then is Wallace's future over the next three years?  Probably something very similar to this season.  Wallace should remain one of the best offensive rebounders in the league.  His streak of 100 steals and 100 blocks will almost certainly continue.  He'll remain the best weakside defender in the NBA.  But, he's likely no longer capable of being a dominate defensive rebounder.  Anyone 6-11 or taller will continue to give him problems.  His days of winning the DPOY over.  If Wallace had turned out to be the 11 win player he had been in Detroit then he would have earned his contract.  11 wins would have placed him among the top ten players in the leauge.  Instead Wallace produced a solid 8.6 wins, which places him with Tyson Chandler at the tail end of the top 30 (Look where the Core ranks).  I hope that Wallace has a resurgent season as the public pressure shifts to Deng and Gordon and their contract extensions or lack there of, but it's likely the Wallace we saw this season is the Wallace we'll see next season.