For me its mostly the tight integration of all Apple Products (a la ecosystem)
iMessage - I send/receive almost 200 a day and the ease of just picking whatever device I have and continuing is awesome
Facetime - Moved all my family members abroad to Apple and getting them to ditch Skype ... just the other day I was talking to my daughter from Office and she said she wanted to see where I sit... no stupid opening any app or login, I quickly tapped Facetime and voila we were good to go
Photostream - (Even better with iOS7 now because of Shared Streams) - Best way to share photos amongst Apple devices ... I share a Stream with my wifey and quickly upload good moments from daily life ... instant notification and viewing
Camera - I still find it to be the best point and shoot out there with single hand use
Size - Its just about right, even though I can do with another 0.5 inch as I have long fingers
Style Appeal - The only phone I believe to match the style is the HTC One and thats Android, so will leave it at that
Price Factor - In Singapore, I purchase iPhone outright and even now almost at brink of 5S launch can still get back 70% of the cost back on sale. No other phone does that here
...
time to rest .. will add more when it comes to my mind