Out of the many features absent on the iPhone, I miss voice dialing and quick contact access the most.
With Windows Mobile smartphones, you can type on the keypad and then the contacts that contain the letters associated with the number will be shown. With the iPhone's built-in contacts, on the other hand, you have to either fumble through the alphabet or type the person's name in the search field, which you have to scroll all the way up to see.
I was very happy to run into Thumbtacts, a contact-management application from Kannuu. It's a simple application that at start-up displays four random letters in four buttons. You then can change these letters by tapping on the middle "More" button, or by tapping on one of the letter buttons to narrow down the contact names. If you keep tapping on the buttons that display the narrowed-down names, it will lead you to the person whose contact information you want to view. All in all, this requires quiet a bit of tapping, but at least it's much more convenient than the iPhone's built-in contacts.
Thumbtacts is far from perfect, though. After a few minutes of trying, my colleague, Stephen Shankland, and I found a list of things that Thumbtacts could improve. This list includes the capability to enlarge text size, retain the last search each time the application starts, better contrast (for use under bright sun), and faster search and load times. The application takes about two seconds to do a "Processing contacts..." job each time it starts on my iPhone 3G (I have about 400 contacts) before I can use it.
I hope that all of those issues will be fixed with the new release. For now, this is a great and handy application to help you browse your contacts, though probably not fast, and just with one hand. The best thing is that Thumbtacts is free, and you can give it a try now at Apple's App Store.