There has been a strong movement in the fitness tracking industry: wearable tracker connected to a smartphone app via Bluetooth 4.0 LE. Each of these terms are indispensable to the success of the movement -- the wearable tracker allows the wearer to monitor his activities 24x7, the smartphone app allows the wearer to see the results and set personal goals, and the Bluetooth 4.0 LE (BLE) allows the two things to communicate regularly and wirelessly. Had any of these gone missing the user experience would most likely be so bad that no one would use the setup. Imagine you have to carry your smartphone all day including shower and swimming sessions, or have to retrieve the data from the tracker with a cable and charge every 2-3 hours.
I'll start off with the first fitness band I had. It's a Jawbone UP. It didn't have Bluetooth so the communication was done by plugging the band to the smartphone. Jawbone did a good job in making the connector a 3.5mm plug so it worked with the audio socket, and the app was pretty solid. Battery life was good (~1 week) and it looked very nice. I often got compliments for its fashionable design. However a huge flaw was it had almost no indicators except 2 faint LEDs and vibrations. In the HCI point of view it's lack of feedback really set it back.
Next it should provide some functions related to the smartphone. I like the fact that I can use it as a remote to my smartphone, besides providing notifications. That's why Nike Fuelband, Fitbit, or cheaper bands that only have 2 to 3 lights don't work for me.
Then it has to be comfortable and durable. If I want to wear it 24x7 it shouldn't get in my way and can withstand a bit of abuse. That's why Razer Nabu (reported to chip off in 1 week), or the chunky Microsoft Band with 10+ sensors (not available in Canada anyway) don't work for me.