EDITED: I missed something actually quite important (to me)
I haven't gone back through the posts in this thread either. My apologies if I'm posting duplicates.
My project isn't quite ready to jump fully into art, and I've held off on using Spriter until the features are more aligned with my needs. Here are the things that would make Spriter a go-to tool for me:
1) Output Controls: The ability to specify a view around the art that crops to particular areas on output. Defining multiple views and output resolutions would enable artists to generate content more easily. RPG's for example may need a portrait and full-body views of a character, but if the character is assembled in Spriter, it's an extra set of work to frame up and crop later.
2) Gradient Color Remapping: Remapping the color values of a sprite part's texture to a gradient strip (ideally created within the app and saved to a "palette" for reuse. This is a very powerful tool for building visual variation easily and is getting used more frequently in game development.
3) Deformer Bones: Special bones that can deform the sprite part they're linked to in a more granular way than squash and stretch can. This would allow less model segmentation and cool effects like bulging and bending.
4) Depth Mask Output: Generate a depth mask per frame; a greyscale image that renders each part as a flat color based on their sorting. The top piece would be white, the bottom piece black - with shades of grey between. Artists working on 2d games with dynamic lighting, can take this in combination with other textures and create ambient occlusion maps and various other effects. They can basically build all the other maps outside Spriter, but the depth info isn't available until they've combined everything in Spriter (and sorting can be changed per frame).
5) Morphing: Cross-fading sprite textures as part layers, with registration points to enable feature alignment (the texture/UV coords are basically being deformed to accomplish this alignment). This would enable really subtle animations to be built, and possibly with fewer discrete sprites.