AS3 is a lot more different then AS2 because of the extensive use of OOP principles. You can still write AS3 on the timeline, but it's considered best practice to write you own classes. So learning all about packages, constructors, imports and inheritance is a must.
Also the event architecture has been rewritten from the ground up, but is a lot more easier and makes more sense. Might seem like your writing more code to begin with but it all comes together nicely.
A great book to read is "Foundation Actionscript 3.0 with Flash CS3 and Flex" and then you might wanna get "Foundation Actionscript 3 Animation Making things move" Both published by friends of ED.
frankly i'm liking AS3. for years i tried but could never totally get my head around AS2...don't know why. despite the learning curve and vast differences between the two versions, AS3 just seems easier to me. i'm no guru...never will be, but it seems to click. maybe it's due to my s/e background...but i doubt it.
AS2 is more easy. You can do a lot of the stuff with tools and built in library.
AS3 is very similar to JavaScript3. Everything you do in AS3 is manually done.
Oh say... you want to make your square move from right to left without using the motion tool. Well AS3 can do that with simple code.
I would agree with mstrock that AS3 is a lot easier. It might take some time to get used to if you're coming from AS2, but I think you'll like it. The 3 major differences to watch out for would be the new class style, handling events, and managing depths.