The display will be darker and not as bright like comparing the 3GS to Iphone 4.
Yes, the 3GS screen is indeed
not as bright as the iPhone 4 screen.
Newer high-resolution screens are actually more efficient than older, low-resolution screens due to an increased aperture (the area of a pixel which actually lets light through, pixels are not little squares), as well as improved materials, reduction of layers, and better LEDs.
The iPhone 3GS used TFT display technology while the iPhone 4 used significantly higher quality IPS technology.
TFT stands for thin film transistor. All smartphone screens, including AMOLED, use TFT technology.
Well, they will be blurry, because they would need to be upscaled.
For example - take current apple.com webpage - its prepared for aproximately 1024px vertical resolution.
Comparing apple.com on an iPad 2 at full landscape width to an iPhone 4 showing the same page at the same physical scale, the images look indeed a bit blurrier on the latter. Mostly because the upscale filter is simple bilinear. But in my eyes text crispness and lack of any screen door effect are far more important.
Interestingly, repeating the same comparison with the iPad in portrait orientation (thus with slight downscaling), the iPhone is the clear winner even with images. A higher resolution screen is good news for those who prefer portrait orientation for web browsing (most pages are taller than wide).
So 4 retina pixels are the same physical size as 1 non-retina pixel, right? If a scaler just doubles every pixel and maps it accordingly, the image will exactly the same. It's only logic.
It's not how the eye perceives it, though. Seen from a typical distance, the pixel-doubled, resolution-doubled image will look somewhat sharper and more pixelated. Four dots of light (considering one subpixel color) spread over an area leave a different visual impression than a single, bigger dot of light.