Having had experience in SharePoint2010, IBM FileNet P8(Content Engine) & FileNet IS 4.0; I strongly feel that one must do a comparative study of the Architectural Blocks upon which these products are designed.
Its indeed a great feel to get to know how the products are built-on, their limitations and their strong selling points for which they stand in the industry. ECM is such a huge domain that there exists several other well known products as well Documentum, NewGen, HummingBird, etc. just to name a few to which I won't be touching upon in this post.
Below is the SP2010 Architecture diagram..
Below is the FileNet P8 Architecture diagram..