For me, in that particular time frame, I felt Apple seemed in its prime from late 2010 to mid 2011. The iPhone4 and ipad2 had launched, there was no equal to the ipad2 at the time, and the iphone4 running IOS5 and iCloud just seemed complete. I felt the 4s, while a nice upgrade, lacked the oomph the iphone4 had when it was first released, and IOS5's notifications made the system seem complete. It didn't help that Siri was buggy as hell either.
From then on, while future versions of IOS would continue to add new features, none of them seemed as "necessary" or integral as multitasking in IOS4 or notifications in IOS5.