The people he faced can be broken into two categories.
1) people who are under hypnosis, ie espada and captains. He is able to cause grimmjow to fall to his knees with his spiritual pressure alone becuase grimmjow believed it was there. the mind is an incredible organ. there have been many documented cases where people truely believed that they were sick and caused themselves to react to a sickness despite them not being sick in the slightest. if you believe you feel a suffocating pressure all around you, your body will react in such a manner. this applies to all aizen's encounters with everyone who was under hypnosis( other examples, hitsugaya(sp.), wolf/fox dude, and even doc lady(i know thier names just too lazy to look up spelling)
2) the oddity in all of these is the second category, people who are not under hypnosis yet are still destroyed, ie ichigo. This one actually has a pretty simple explaination that many people miss. We all know ichigo has a disgustingly high amount of reitsu(sp), however what people do not seem to remember is that the whole soul societ arc happened in a relatively short amount of time. in fact, remember that all of the soul society pelople were amazed at how fast ichigo was getting stronger and kept getting healed. however, by the time ichigo faced aizen he had fought 1) kenpachi(crazy strong) 2) byakuya(crazy strong) 3) renji(pretty damn strong) 4) ikkaku(just crazy). He was nearly killed by kenpachi, byakuya and renji. on top of that he took on the execution bird thing and 3 leutenants(i consider these insignificant) but what was not insignificant was that ichigo spent three days in training which almost killed him yet again. The amount of exhaustion ichigo felt when he finnally fought aizen would have been insane. no crap ichigo would not be able to touch him, a fully rested elite captain. that is why aizen messed him up so badly.
-side note, i also believe that aizen is some type of hollow/shinigami mix but that is a side note since it has not been proved either way.