I'm satisfied with the PRS-505. There's not much I'd change.
Faster page-turns would be nice but it's fast already.
A better EPUB rendering engine would be nice as I'd have an alternative to LRF.
A Wi-Fi adapter (which you can disable) would be nice for transferring books but more of a luxury.
I'd actually remove the MP3 player and Picture viewer as I've never used those. I don't see the need for a dictionary as there are not enough words that I don't know.
The ability to make notes would be nice for proofing books but if it means a blurry screen then I'm not interested.
A replaceable battery (which you can easily buy) would be a huge plus point too as I think a lot of devices these days are limited by the battery's inevitable decline.