I have one of the first tablet PCs. It ran Windows XP Tablet PC edition. The tablet functionality was tacked on. The cursor also lags WAY behind the stylus. I've been amazed to see that this lag was never corrected no matter how fast processors got. Additionally, there was very little software designed specifically for Tablets, because the market was never very big. Beginning with Windows Vista the tablet functionality was built into the OS, but it never stopped feeling tacked on, and software support remains negligible.
Your mention of convertible tablets actually illustrates my point. Whenever an application works better with a keyboard and mouse/trackpad, just flip the screen around. But since 99% of the software is designed with a keyboard/mouse in mind, you end up using your device's tablet functionality about 1% of the time. I know many people who have had tablet PCs and for ALL of them it has been the same: Seemed really cool, but it turned out that I never use it in tablet format.
The iPad is the opposite beast. If you want a keyboard you have to buy it separately. Every program is designed specifically to be used on a tablet.
Also, to answer your question, you don't write on an iPad. There's no stylus (there is actually, but it's irrelevant to this discussion). You only type. And the on-screen keyboard works far far better than anything that Microsoft has done. But more importantly is that it's not just the on-screen keyboard. It's the entire user-interface. It's how the on-screen keyboard works with the applications, which have been designed from the ground up to be used on a tablet device in every single way. Mobile Safari is designed for a tablet in every way including methods of input. Internet Explorer is designed from the ground up for a PC with a keyboard/mouse and repurposed for Tablet PC devices by tacking on a few poorly conceived, poorly executed new Tablet features.
In the end, it's just experience. I've used them both, and they provide vastly different overall experiences. The iPad is the far superior experience in almost every way.