Brett Terpstra loves apps that are designed with a unique look but that use familiar controls. As a design philosophy, his ideal applications are both easy to use and fun to look at. Delicious, if you will. Cookie-cutter app elements may be great for prototyping an application's infrastructure but to him an app that really stands out has a unique visual voice.
He really loves Tweetie 2's ability to refresh your Twitter stream by scrolling to the top (with a tap on the title bar, of course), then pulling down and letting go. It's non-standard, but so intuitive that it should be. That's the kind of attention to detail that makes him feel warm inside.
For me, it's all about putting yourself into the mindset of the user. Were features put in place because they need to be there or because the developer thought it would be cool? Is the developer really understanding how real people might use the app? Take the iPhone. Is there a switch that lets you disable autorotation when you're using a game while lying on a couch or in bed? Because that's how real users use real games. And if the developer hasn't thought about details like this, they haven't hit that design sweet spot of matching real world needs against programming limitations.
Chris Rawson loves well-designed icons. They should give you some indication of what the app does, so you can launch it with barely a glance. Want to use multi-touch controls? Make sure those controls are intuitive and that they provide an innovative use of the technology. In his opinion, Convertbot is a great example.
Remembering already-set preferences is another hobbyhorse. Take iPhone apps that connect with the Mac over Wi-Fi for example. A brain-dead simple initial setup should be followed by only needing to push a single button (if that) to connect on all subsequent connections. And as for a finishing touch, he feels that well-polished graphics shouldn't look like something squeezed out over a single afternoon by someone with no artistic skill whatsoever.
Well designed, easy to follow instructions are key for David Winograd. In the best of all possible worlds those instructions should even include user interaction, whether via Q&A or by modeling, to make sure the key points get through to the user. In his opinion, Irene's Spirit has the best tutorial he's ever seen in an iPhone app. If the app were as good as the tutorial, the world would probably implode.
These are just a few key points from our team. Add yours to the comments below. What are the key elements that make an application, whether for Mac or iPhone or any other platform, shine for you?