Slashgeo reports yet another new Open Source GIS tool, noting "Open source GIS software is crowded with options: QGIS, uDig, gvSIG, OpenJUMP, MapWindow, GRASS, OSSIM, etc."
Yes, it is - and the question to me is: "Why so many?" - I understand the arguments for a diverse software ecosystem, and I even agree with them. But with the possible exception of GRASS, most of these packages implement the same basic functionality without implementing the advanced functionality that is necessary to seriously take on ESRI. Plus, the UIs are almost always terrible, which makes the problem even worse.
So, in that spirit I'm listing a few things that I think are missing from most, if not all, OS GIS software, in no particular order:
1. Topological data model (the only exception I can think of is GRASS)
2. Editing, especially centralized, version controlled editing.
2a. Which means you need a topology model in your database
3. UIs not designed by software engineers. (Well, at least ESRI has this problem, too.)
4. Quality cartographic tools.
5. COGO tools
6. Metadata editing tools
7. Scriptability (GRASS, being command-line based is fairly scriptable, I know of no others that are.)
8. Runs on my Macs.
9. Comprehensive interface for building extensions
Well, those are a few that come to mind right away. Make more suggestions - maybe we can turn this into a manifesto.