How come LimeWire doesn't come with an inherent library data structure like iTunes? I.e. I wish I could access my music collection directly from LimeWire and create playlists. It would be great if playlist creation can be done within the single application without opening any folders?

There is a clear business reason for this feature. Consider that LimeWire has been downloaded about 70 million times? That's a huge 'market share' (if you will). Why end the typical user-experience at just downloading? Right now, it appears that the average user downloads a set of desired songs, shuts down LimeWire and fires up iTunes to listen to the songs and engage in all the other iTunes interactions that are commercially benefiting Apple. Imagine what leverage LimeWire could have by offering even just a slightly more advanced player (e.g. library support) and keeping users. In fact, this would be something to particularly consider after the Orchard deal if it was ignored in earlier versions.