Joel Hill writes:

>>My question is: What should I look/listen for to make its
>>"masterpieceness" more evident to me? I am not coming new to Mahler - his
>>3rd is one of my favorites.

and Mimi Ezust writes:

>Then, buy yourself a few of the less expensive cds.  Compare them.

Dare I say you will find help with choice of recordings at the following
site:

   http://www.musicweb.uk.net/Mahler/Daslied.htm

and Joel, you will find a short introduction to the work there also.
Hopefully, an explanation of its "materpieceness".

However, as with all great art, pinning down the "why" is fraught with
difficulty.  I know of one person who couldn't listen to Mahler without
being sick.

Tony Duggan, England.
Mahler recordings survey
http://www.musicweb.force9.co.uk/music/Mahler