If you depend on Quicken for Windows, then obviously, the iPad can't do that.
If you have a Garmin device and you want to update its firmware, the iPad can't do that. As a matter of fact, it can't update the firmware on any device. So any gadget, like an Escort Radar detector, a Netgear NAS, a Logitech Harmony remote control... - out of luck.
You can't run any Windows or Mac software, obviously. So the client version of DVD Profiler (an awesome iOS app) only runs on Windows.
Need flash? Nope.
Need access to your file system, you know, like to copy files around, upload them to websites, attach them to emails? Nope. There are 50,000 work arounds, but don't try to reply to an email and attach a document. Can't do it.
And on, and on, and on, and on....
iPad is a great device. And if all you do is check email and browse the internet on non-flash sites, then it can replace a computer for you.