From my Treo 650:

- System-wide search - and the N900 has no calendar search at all. This is the top issue for me.

- A few PIM issues such as the lack of task description and alarm setting granularity.

- Proper IR port. It was handy for copying infrared remotes, although I hardly use that anymore.

- Slick UI. On the Treo you could do everything through the keyboard OR the touchscreen (with Graffiti Anywhere you could write with gestures). On the N900 just one or the other won't do the job. The Treo's interface felt like it was highly optimized for everyday use, everything just felt natural. But to be fair the N900 is more of a mobile computer than a PDA like the Treo was.

- Battery life. The N900 will barely make it from morning to night in a best case scenario, and if you use it like a laptop it runs down the battery like a laptop.

Overall the Treo felt like a better PDA/smartphone, but I still prefer the N900. Its better browsing, multimedia and computing capabilities more than make up for its shortcomings as a PDA/smartphone.