Most comparisons below are with OSQA. I've never tried to install Shapado so never compared it with askbot on the same hardware.
- Faster response time (well under 1s for most pages) because I rewrote all templates in a speedier rendering engine - Jinja2. Template rendering time decreased at least five-fold. Those templates will also help make custom skins - this is the plan for the rest of December.
- More reliable code, there are ~350 test cases in the automatic testing suite. Last time I've checked there were none in the other projects.
- Easy to install and instructions do not depend much on platform. Askbot installation uses standard Python packaging solutions.
- Active development - activity in the repositories can be easily compared. I've translated the Chinese CNPROG in the summer 09 and kept improving the program ever since.
- Easier to deploy with other Django apps for those who are interested.
- More of valid markup. I went through all pages I could with the W3C validator.
- Bugs normally fixed on the same day and support is less formal. You can report bugs right here on the forum.
Features that are probably absent in OSQA:
- Search by multiple tags and text query. The query can be modified while tag selection is kept. Also there is a "search as you type" feature - more of a toy but is kind of nice.
- inbox feature - the "red envelope" that pops up when there are new responses or flagged posts (for moderators)
- command line tool to merge, split and rename tags (will be added to UI as well)
- probably finer grained user level - admins, mods, regular users, suspended and blocked. All tested.
- ajax retag
Features absent in Askbot:
- re-posting comments as answers and similar
- no votes for comments yet.
There of course will be more good features, but I think it's better to get things done first and then talk about them. However, starting next week there will be a redo of templates and related things to make site customization easier.
As for the smaller user base - you see the project is still very active. Larger user base will only add motivation.
Posible duplicates: http://askbot.org/en/question/6941/what-is-the-different-askbot-from-osqa?answer=6945#answer-container-6945