I would like more speed but I do not like the form factor of the current lineup of PowerBooks. I really like having two internal hard drives like I have in my PowerBook Pismo. I use one for my main drive and the other for backup. Every night the machine automatically backsup to the 2ndary drive. Weekly and monthly it makes backups to a network device.
Another thing I like about the Pismo is the plastic case. It feels better than the slick new metal cases and takes a lot more abuse. I've dropped this machine several times from chest height. By accident - I'm not a computer abuser! Not intentionally anyways! One time the machine was running. It didn't even hiccup. (I did! *gasp*)
I also really love being able to have two batteries for long trips (swap out the backup hard drive in the right bay, slip in another battery). This means with my three batteries I was able to get up to 15 hours of on the road time without needing a wall socket.
But, I would like more speed. My sons's new iMacG5 2.0GHz 20" screams. very fast. I would love that in a PowerBook, preferably dual-core and speed reduction for power savings. But, I don't want the power drain or heat. (Care too cook eggs! Actually, the iMacG5 isn't that hot. They would be a little runny.

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I would also like more RAM. Right now I only have 512MB of RAM. I had 1GB for a long time but the Pismo would overheat and turn on the fan in the summer a lot. With 512MB it rarely turns on the fan.
Lastly, I like the black case of the Pismo, not the silver or white of the newer machines. The black plastic case hides dings, scratches, finger prints, etc better.