When it comes to features:

• Block calls - per contact/number/pattern. Either with auto-silence or auto-reject, preferably both.
• Block messages - auto trash SMS/MMS/IM/email with similar settings as for blocking calls.
• Bluetooth HFP, PBA/PBAP, SAP, SIM, rSAP and AVRCP profiles for allencompasing in-car support. MAP has already been mentioned, but the aforelisted are as equally important. Or at least allow us to hack the BT stack like on the N900 - to this day I can't get my N9 to behave nicely with my car.
• Browser history and cursor hover mode like in the MicroB on the N900. Also, a quick way to scroll to the top/bottom of a page.
• Debug/verbose mode for every built-in application so that we can more easily and accurately report possible bugs. To this day I can't figure out why the N9 refuses to connect to one particular IMAP server as it just throws a simple 'Error connecting...' message.

When it comes to specific apps, apart from the already mentioned:

• Swype keyboard (should be built-in if you ask me)
• Billboard (from the N9) app but with more control and abilities for which support would have to come from the SailfishOS itself. Most preferably a blend between the Billboard app and Nokia Sleeping Screen from Symbian.
• Already mentioned but +1 for Fuelpad (from the N900, never made it to the N9)
• A good source-code editor, MeeTxtEdit at the very least
• CloudGPS
• Marble
• A fully featured XBMC remote