In what ways could Best Buy have improved the screen in store? Some have said on here it's not as good when you get the device home. Not sure I'm on the fence with that claim though.
Best Buy can't improve the display in the store. If anything, a store display model will look worse. It will be more likely to have scratches and smudges all over it and the bright overhead lighting in stores will typically create more reflections on the surface of the display.
At home, you'll likely keep it clean, not abuse it and not view it under direct overhead lighting.
The only thing the store might do is keep the brightness as the top level. Their models are always hooked up to power, so maintaining battery life isn't an issue. In the real world you'll rarely go over 50% brightness.
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The mini's display isn't bad. It's just not Retina.
Think of it like this: A Retina display is like a Ferrari, a standard HD display is like a BMW, an iPad mini display is like a Toyota.
Is there anything wrong with a Toyota? No, it's perfectly adequate for most anyone, but Ferrari's and BMW's are certainly nicer.
Much like those choices in cars, there are tradeoffs as well. Is the Retina display awesome? Yes. But to get it, you need to pay more money, have a larger and heavier device and have a longer recharge time. You also of course get a faster processor along with those negatives--and a larger display, which some prefer.
At the end of the day, it's up to you. If you've played with it 3 times and have no complaints, you'll likely be perfectly satisfied with the mini.
Remember too that not everyone can even notice the difference. If you don't have perfect near vision (no one over 40 does) in all likelihood, you physically cannot discern the individual pixels anyway--thus making the Retina display a purely academic feature.