We have recently launched a large government site re-written using drupal 7 (> 3000 nodes, high traffic). Having gone through this development, we had to learn a few things:
- due to views only recently being made stable, a lot of modules that depend on views are still unstable. Even so, the more popular ones work okay
- we had to port some modules that we use a lot, that had no D7 branch. An example was Apache Solr Panels
- features in D7 was shaky for a while, but now is fairly stable
All in all though, Drupal 7 has awesome new features, a whole new database abstraction layer, a wicked states API, image handling in core. As time goes on contrib will keep getting better and better for D7, whilst you will find shortly I would imagine that none wants to make new features for D6 contrib.
If you want to check the site out, visit health.govt.nz
