I'm new to this language.
But I see lots of people prefer as3 to as2
AS2 is a much more dynamic language, which is far more open to things as AOP, mocking or meta-programming ... also AS2 is fully prototype-based ...
AS2 as well has proper classes and interfaces ... it's a language that can be well compared to ruby or other dynamic languages, only it supports compile time type checks ...
most people never understood AS2 and don't understand it until now ...
if you are seriously interested in using AS2, I suggest using MTASC as compiler and FlashDevelop as IDE (already comes with MTASC bundled) ... also, you could use Haxe to have an expressiveness superceding that of AS3 and compile to AVM1 to have the dynamicity of AS2 ...
the 2 most important features of AS3 are:
after 3 years of AS3 experience I'd personally advise you not to learn AS3, but rather Haxe... I'm hoping for Adobe to make a move some time soon, but currently the language (not to be confused with the API) has stalled since its very release and all language feature requests on Adobe JIRA are plainly ignored.