* how does the display compare (brightness)?
I think they are very comparable in brightness. The size of the Nokia screen is really nice. The Nokia has a slight "sparkle" to the screen; especially on whites. I don't know if the screen is slightly transflective or what. It's hard to explain without seeing it, and doesn't show up on photographs I've taken. It's kind of like the "sparkle" of metallic car paint. Also, you can see a slight curtaining effect where the backlights project onto the screen. None of these things really take away from the screen though. The Nokia screen is extremely pleasing on the eyes.
* is the screen resolution of 400x800px a recognizable improvement?
It's actually 480x800px. So, it's the same height as the Zaurus, but 160px wider. Yes, the screen resolution is really nice.
* how good can it be used without a built-in (clamshell) keyboard?
This is definitely a weakness of the Nokia. I often use the Zaurus on-screen keyboard because I'm in tablet mode and don't feel like opening up the keyboard, so am used to tap-typing. Unfortunately, tap-typing on the Nokia is a much worse experience than the Zaurus for me. For one thing, the keyboard takes up the entire width of the device, so the keys are really spread out. This means you have to make large movements to get from one key to another, and this really slows me down. Also, they left out the right shift key, so you have to go all the way to the left side of the keyboard to hit shift even when you are capitalizing a letter that is on the right side. Another problem is that it seems to not always catch every letter if you are going really fast. The auto-complete feature just slowed me down, so I finally turned it off.
I think the Nokia on-screen keyboard is designed for very casual tap-typing.
* what about operation time?
The battery on the 770 is a bit smaller than the Zaurus, but the processor is also less power-hungry. Overall, I think run-time is pretty similar. I'd say 5-10 hours depending on what you are doing.
* device handling (thicker, thinner, larger, smaller)?
I really like the form factor of the Nokia. It is very thin, and bulges less in a shirt pocket than the Zaurus. Once the Nokia's metalic slip case is attached, the Nokia seems very sturdy -- I've heard a Nokia rep flung one across a room to demonstrate this at a press conference. The build quality is top notch.
* how stable and complete is the software compared to a Zaurus Sharp/Qtopia ROM?
The Nokia is not nearly as stable as the Zaurus. Out-of-memory problems are the main issue. It can be pretty annoying. You can really see the extra several years of development that has gone into the Sharp product at this point. However, each new ROM Nokia sends out makes the situation much better. This is a beta level system, but will improve dramatically over time. Also, if you are just running one app at a time, it's quite usable. For example: I ran a GPS mapping program on a 9 hour drive, and it performed perfectly (on the return trip as well).
* how is performance compared to a Zaurus?
At this point, the Nokia is definitely slower. Nokia chose to use a multi-core processor (CPU+DSP) that runs at a slower clock speed (220Mhz vs. 400Mhz). This means that apps must take advantage of the DSP to get up to the same speed as the Zaurus. So, as more apps and libraries take advantage of the DSP, then the system will get faster. The Nokia certainly isn't super slow though. I found the PDF viewer to take the same time between pages as qpdf on Zaurus, for example, while doing a better job of rendering the PDF.