Looking at Shaq's +/- in Cleveland - in a vacuum - tells a negative story - he cost the offense 6 points per 100 possessions, and was neutral on defense.
Putting those numbers in context, however, tells a different story.
When Shaq was off the court, Varejao was on. That puts the ball exclusively in Lebron's hands. Lebron is better at offense than Shaq... mystery solved.
The flip side is, Cleveland's defense did not see any noticable decline in efficency when Shaq was on the floor. Varejao is considered a very strong defender. This is a good story.
Finally, the biggest difference between Shaq's next season and his the previous two will be his competition - he will be defending and dunking on second teamers next season.