The limiting factor on a phone is what you can wrap your hand around. This thing looks so wide it couldn't be operated in a single hand. I notice that a lot of BB users are two-handers though, perhaps because of the keyboard, so maybe it isn't a problem in their target market.
Square screens aren't a Blackberry issue - they've had square screens ever since they used that godawful pager protocol.
The thing is, it was fine to use a blackberry two handed because they strongly considered the single-handed use case and developed around that.
The UI, etc, are designed to be used single handedly - back when it was a 160x160 screen, they had the scroll wheel that let you go up down and select. And things modernized, they used the central click-ball as a virtual 5-way controller (up/down/left/right/center) and as a left/right up/down scroller. That became an IR sensor (not unlike an optical mouse sensor) later on.
And that makes a HUGE difference in usability - it didn't matter you needed two hands to type - you could navigate and figure out if something needed an immediate response or could be deferred on the go.
And that's what big screen Android phones lack - they don't take into account the single-handed use-case. They assume your hand can reach all four corners of the screen at all times, which generally requires two hands.
Well, if single hand use is important, then you either have to use a smaller screen (impossible on Android as small screen phones tend to be crappier as no one seems to innovate at making flagship phones with decent high-res screens 4.5" or smaller), or use Blackberries or iOS.
Perhaps Google needs to rethink how we interact with our phones and impose a new paradigm for large-screen phones where all controls need to be reachable on the left or right (depending on handedness).
Because that's one way Blackberry excelled - it was a two handed phone that let you use it as a viewer singlehandedly.