I totally disagree. Snape had EVERY right to shake Harry like a rag doll. Can anyone fathom why? I'll tell you.

Okay, first of all, we all know that Snape and Harry don't get along.
But Snape gets the upper-hand because he's a teacher and Harry's a student.
When he had to leave the room, Snape specificly told Harry not to fiddle with anything, or something like that. Harry didn't listen. The stupid boy went over to the pensive and went into Snape's personal memories! I'd be incensed! Hopping mad!
Harry happens to stumble across Snape's worst, most humiliating memory that he can remember. In it, James makes a complete fool out of Snape in front of many students, one of them, Lily (there's some speculation that Snape liked her, so that would make the experience that much worse).

After this experience, Harry is feeling ashamed of his father's performance, but Snape dosn't know that. All he knows is that James was a monster who was bullying to the point of cruelness. He obviously sees a lot of James in Harry, and thinks that Harry sided with his father in the memory. Harry talked up to Snape in the 3rd book ("shut-up about my Dad!") and Snape probably still remembers that. He sees so much James in Harry, perhaps he sometimes forgets that Harry is not James, a mistake that Sirius seemed to make though for different reasons.

So, Snape walks back into his office, and finds that Harry, of all people, is peering into his most humiliating memory. He is outraged (for good reason). He grabs Harry, pushes him across the room and roars at him in his anger.

I don't think that Harry deserved it so to speak, but knowing Harry from Snape's viepoint, yes I can see why Snape became so angry and I don't blame him at all for his show of anger.

And yes, there are probably a mllion people who disagree with me, but that's my standpoint anyway. ;)