Get Custom HiDPI resolution rendering on non-Retina Macs

Images and icons look about the same because of integral scaling. But text in non-Retina apps looks clearly worse. The difference is that on a non-Retina display, text uses sub-pixel rendering. But with Retina, old apps use full-pixel anti-aliasing instead. (Those full pixels are then multiplied perfectly with integral scaling.) Not quite sure what Retina text on a Retina display uses, it's hard to eyeball.

Separately, other resolutions look pretty good. The highest is 1920x1200. The trick is they are also rendered in double-resolution (3840x2400) and then scaled down for the display (2880x1800). Because there are so many tiny pixels, the result is acceptable, but the optimal setting is definitely sharper.

So Retina apps at 1920 look pretty good, and non-Retina text at 1920 has smaller and slightly fuzzier full-pixel anti-aliasing.

It depends of what you mean by "will look the same".

It looks the same only in the way that the elements on your screen are displayed in the same size as if it was a 1280x800 screen.
The aim of the retina displays is to display more details, to do that, what it does is that it displays things at the same size a half resolution screen would have done but uses all the available pixels to show you more details, nothing is better than an example to explain you what I mean :

In fact, the pixels of a retina display are designed to be small enough so that you retina won't be able to distinguish them at a standard use distance, that's why it looks as sharp as a printed page, and that's why a retina display will never look the same as a display that would be natively 1280x800 pixels, actually it should looks much better.

Now let's answer to your question ! You didn't explained if it always looks blurry or only on some applications/websites.
I will assume that it does not always looks blurry because it shouldn't.
As I explained, although they are displayed in the same size than on any other display, the elements are "constituted" of 4 times more pixels which means that an element that would have been 10x10 pixels (100 pixels) needs to be 20x20 pixels (400 pixels) to take advantage of all the available pixels ; it looks blurry if the element hasn't got a resolution that's high enough "to fill all the available pixels", in fact, this is exactly like when a picture is pixelated but here it looks more blurry because the pixels of your screen are extremely small.

There should be more and more retina ready websites/applications in the future (in think that in something like 4 years most of the websites will be).