In the App Store; there is a massive amount of Apps that have not yet been updated to support Mavericks. Really annoys me when I see lazy developers (even the likes of Rockstar have not updated their apps).
What specific feature would you like to see these lazy developers to embrace?
Most of the updates to Mavericks is under the hood, there's little to change for the consumer.
All of the apps I use or buy have worked flawlessly and that's a mark of a good OS and application.
There is a LOT of games on the App Store where people are saying in the comments that it suddenly doesn't work when they updated to Mavericks. CCleaner on the App Store no longer works...