After upgrading my first mac (powerbook) to another powerbook, then to an iMac and finally to a Mac Pro, I realized five years of using the Migration Assistant had finally run its course. Various basic parts (mostly Keychain Access) of Leopard stopped functioning properly and since everything ran great on my new Macbook Air, I decided it was time to backup, format, and reinstall fresh on my main Mac Pro.
A few hours after upgrading I installed Firefox and my most often used apps like Transmit and Textmate. Every few days I realized I needed one more app so I'd download and install it. After a week or so, I was pretty much done reinstalling.
Last year I wrote about doing as much as possible using online apps and how I found it really handy, so today I looked at my Applications folder to see how many things I've installed aside from the default Mac apps. I counted 11 applications total outside of iLife and iWork. It includes a couple proprietary things I need for installed hardware (like the wonderful ScanSnap) but it's mostly the basics (Firefox, Transmit, etc) for doing my everyday work tending MetaFilter.
The thing that surprises me is that I reformatted my computer about six weeks ago, and I haven't felt like anything is missing since. Thanks to a combination of almost all my work being done online and the great set of built-in functionality of OSX, I can get by on an almost completely clean system.
Ten years ago I had literally hundreds of apps on my Windows box, and I feel like I was constantly needing more.