I just noticed that Optimal plays audio from within the outline (or you can download the enclosure).
So now not only can you read the contents of a feed, you can also listen to it…screen shot.
Bitty displays a “play” button but it opens your PC music player.
Grazr lacks a play button full stop.
Something I noticed is that Grazr and Bitty go one more step by showing full-text of the post, whereas Optimal stops at the title of the post.
Also Grazr lacks to show the OPML URL of an inclusion, whereas Bitty displays it at the bottom of the screen, and Optimal offers it if you click on the red arrow.
[ADDED 04/04/06: Grazr announced it is adding an audio player and open the URL of individual nodes (handy for OPML inclusions).