The single best feature which contributes to this is the instant on of the hardware. I mentioned this in my hardware article (http://www1.clearlight.com/~oakley/hardware.html) in the context of what makes the MessagePad a great piece of hardware. I'm mentioning it here in the context of what makes the device easy to use.
As an example, suppose I need to jot down a quick note; perhaps a ATM withdrawal or a phone number. Open the lid, press the power switch, and faster than I can grab the pen the device is waiting for my input. It's amazing what this does for makeing the device so accessible. I think even waiting 5 seconds would diminish the utility of a handheld, so unless it's on instantly it's just not good enough. I've never once thought "Nah, I won't enter it in the Newton because it's just too much trouble". It's never too much trouble to enter data into the Newton.
Ok, so great. We've beat that subject to death. So, let's talk about when the machine is off. It happens. As much as I love the little beastie I have to turn it off sometimes. Yet, the Newton is still paying attention. The other day I was dropping my son off at daycare when I heard my Newton's gong, signaling it had a reminder slip for me. I popped it open and it reminded me to run an errand before I went home. This is absolutely great! I didn't have to remember to check the Newton to see if I had forgotten anything. It knew to wake up and alert me to something.
This is an absolutely indespensible feature for me. I don't have time (or don't want to make the time) to check my todo list all the time. I'm interrupt driven. So, if I have a task I just schedule a reminder at the appropriate time and I don't have to worry about it. The only thing I have to do is make sure I have the Newton with me, and make sure it has enough power.
What else makes the Newton so easy to integrate into daily activities? The filing system. It's so transparent it's almost scary. I'd wager it's darn near impossible to lose data, *any* data, from a Newton. Everything is automatically saved. Writing a note at a traffic light? When the light turns green just flip the power switch off and drive. Next light, turn the Newton on and you are right where you left off. This is absolutely wonderful, and part of what makes the Newton a Newton.
Now, if you are one of those people who just has to organize your data, the facilities to do so are admittedly somewhat limited. But, for people like me who use the thing as an electronic notepad, filing and organizing is just not necessary. If I want to find something I wrote down I don't have to try to remember where I put it; I don't have to remember if it's a business number or a personal event, or whether I filed it under "temporary stuff" or "things to do". I also don't have to remember what program I used to enter the data to begin with. I just tap find, enter a word or two that's part of what I'm looking for and I've got it in front of me quicker than you can say "Bob's your uncle".
That brings me to one final point. I think the software that is shipped with every Newton is precisely right. When I wrote my article about the global search facilities someone said "you can get an application to do that for Windows" (or something to that effect). The beauty of the Newton is I don't have to go to a third party for basic functionality. Same goes for a notepad and handwriting recognition. You can get a notepad, and you can get handwriting recognition for Win/CE, but it's not bundled in. It's not a basic feature you can count on.
With the Newton there are so many basic, fundamental features that I take them for granted. And perhaps that's the best testament to the usefulness of the Newton -- it is so easy, and the interface so transparent, I just take it all for granted because it's so simple to use. I never feel like I'm "using a computer". It's just a notepad. And an address book. And an appointment book. And it's easier to use than the individual paper counterparts.