The big new feature of 1.5 is a new brush tool, along with the "Brush Designer". The designer lets you make new brushes and change existing ones. It also provides its own little canvas to try out your brush, and as you tweak the settings it will dynamically update your last brush stroke. It's pretty fun to play with and you can find it under the Window menu, or by clicking the new brush selector.
If you've got any feedback, use the built in reporting tool under the Help menu. And I'd appreciate any you've got!