1. Different brushes
There are 3 brushes that are available to "paint" with, and I'm not sure if it's merely a different brush type or if it's supposed to simulate a different paint type, but I can tell that they're different. I won't pretend to know a lot about art mediums, so I will leave it alone. You can also change the size of your brush, from 1px in diameter to 64px. The same goes for the eraser.
That's right, "colours", with a 'u'. The app uses "Color", but I'll forgive the author since he includes the entire spectrum of RGB colour choices, with varying transparencies to choose from. A little colour picker lets you find a colour you've already got on the canvas.
How awesome is that? Four little layers to choose from, to move around, fill with colour, merge, or delete. You can also import one of your photos as a layer, even awesomer!
Just like any other iPhone/iPod Touch application, pinch to zoom in or out. I discovered this after a few paintings, and it makes a big difference. This is very obvious.
5. Brush stroke log
You can undo and redo nearly any brush stroke you've painted during the session, and even better, you can download the image as a Brushes filetype, and use the viewer available on their website to turn it into a movie of your painting, from start to finish. I imagine this could make for some very interesting movie art.
In the corner of the Gallery, they give you a little wireless/RSS-type icon, which gives you a URL to try, if you're connected to a wifi network. This is how you download your paintings either as .png or whatever format the Brushes file is in (I haven't tried it yet).
My paintings, so far, are in my Picasa album, and I hope to add more to my fine portfolio. Here are a couple of my favourites:
The left is the first painting I did, the right is one I did after I discovered you could zoom in. It makes a suitable iPhone background image.