Whatsapp - I think in terms of desperation that we gauged from Twitter users about missing apps on the N9, Whatsapp is right up there. The koombayah of all phone devices helps friends message each other for free over the internet rather than through sms. This program overcomes the silo treatment that Blackberry and more recently, Iphone has tried to impose on its users. We hope that this app comes to the N9 fast because whilst we sit pretty with the most beautiful phone (period), currently we are feeling a little left out of the camp fire right about now.
Google Maps - There are times that a certain location cannot be found on the Nokia Maps built into the N9 and its times like these that I open up Google Maps for a second opinion. The Google Maps built in friend finder Google Latitude is also sorely missed.
Quickoffice - If the Nokia developers were aiming the Nokia N9 to be a slick business phone, they certainly missed the mark by not including a Word/Powerpoint/Excel editor with the phone. Many of the Symbian users have grown to appreciate Quickoffice in times when last minute changes needed to be made to a presentation or revisions to speech notes. Quickoffice or something similar to it needs to come with the PR1.1 that we are all waiting for.
That's the list off the top of my head. What apps do you crucially miss when you switched over to the Nokia N9?