I miss the size, weight and relative "disposability" of my 5800XM
I miss Ovi Maps 3, which starts up and has a GPS fix in < 10 secs, and got me from Dorset to London and back today, turn-by-turn.
I miss its sound quality - I did a side by side comparison last night, and the N900 sounds just awful, with no bass.
I miss its reliability - my N900 wouldn't boot this morning, and needed a hard re-flash. no idea why.
I miss the screen quality - just way 'nicer' (indoors), and side by side the 5800 really shows up the N900's yellowy cast. Of course the N900's transflective screen excels outdoors.
Miss hardware buttons, easy one-handed use and 'standard' power connection.

That's about it, though. the S60 UI made me want to stamp the 5800 into the ground on a regular basis.
I love the N900, and it has a very exciting future.
Insanely, I was glad when I got my first (minor) screen scratch because it put an end to my debating about selling it and going back to the 5800. Now I feel I can stop treating it like a baby, and start using it as a workaday tool.