Retina question

You're basically correct for apps that have not been "retina updated". There are some perceptible differences that people have determined is just because of the smaller gaps between pixels.

Also such apps and images displayed by them will look 'fuzzy' simply in comparison to how sharp and detailed everything else is, even though that app looks about the same as it would on a non-retina MBP.

For example, in Photoshop (non-retina-optimized), displaying an 800x600 image with the "Actual pixels" zoom option should actually show a 1600x1200 image on screen, with each pixel of the image filling four pixels on the screen. Depending on how they choose to implement it, a retina-optimized version of Photoshop might actually show that same image at a true 800x600 pixels.

 

Correct. As another example, if you open Final Cut Pro X 10.0.4 on a MacBook Pro with Retina Display, the video viewer will consume 40% of the screen real estate when displaying 1080p video at 50% scale. After the Final Cut Pro X 10.0.5 update that added retina display support, the video video will consume 40% of the screen real estate when displaying 1080p video at 100% scale. So pixel-basd images essentially become half the size they are now.

Presumably when Photoshop is updated, the same will hold true. For the time being, Photoshop displays documents at the same size as they do on the non-retina displays, but the image quality is the equivalent of viewing the document at 200% scale.