Selection and residues, along with some rendering features seem most critical to me (we have stick but it has a rendering bug I need to dig into).
I'd put selection slightly above residues because I think it's critical for copy/paste/clear and moving fragments. (Which I guess also means that inserting fragments is a needed piece.) Indeed, I think selection affects a *lot* of tools and extensions, so that's my preference.
I would like to expose the transfer function editor from VTK, and finish off the volume rendering piece but would put that firmly in nice to have.
Oh, I think I'd call the "transfer function" the color gradient (e.g., blue = positive, red = negative, white or green is 0, etc.) Yes, I think that would be very nice to have, but it wasn't in 1.x, so I'd hardly consider that a showstopper.
Volume rendering.. well, you know that I'm a big proponent and I'd love to see that happen soon. Realistically, I don't think it's critical either.
I'll put the list up online somewhere tonight..