My belief is that the 620's battery isn't powerful enough to cope with the double tap to wake feature so Nokia have not enabled it for the 620. If they had have done the internet would have been flooded with complaints of poor battery life after the Amber update. The omission of an FM radio is due to the placement of the headphone jack or so goes the popular theory. Anyway, it is Nokia's official stance to blame limitations of the hardware to explain away these missing features from their lower level phones.
Even with these missing features the 620 is still a fantastic Windows Phone for the money and you won't find anything like it running Android at this price point in terms of performance and features it already has. My wife has been very pleased with her 620 and hasn't complained once since I bought it her when it launched in the UK.
Indeed it's a wonderful phone. But I guess I gonna buy a Mugen battery for mine, maybe those made for the 520, it's the same size as the original one, says Mugen by email.
My wife has my old Galaxy Mini2, first they promised an update to 4.2.2, then they stay it's gonna stay at the 2.3 :s
They can always add extra stuff in the future, but I guess with did receive another GDR2, so with cut-out features...
I don't know if somebody can activate those features on a 620 and see if double tap and such work....
But in fact they lie to us consumers, cause every check on the hardware done by others, show it's possible to have them...