As a long-time member of the "12MP is enough in most situations" faction, your observations do not surprise me. One question is "how close did you get to the screen? I am guessing that some reviewers were looking from way closer than ome would normally view an image that is almost 2 feet wide, while you were viewing in a more normal fashion.
One comment though: web pages are a poor way to measure text handling, since the fonts used are typically chosen to handle the relatively low resolution of most screens, san serif fonts like Helverica, Ariel and so on. A web page common design rule used to be aiming at a page looking good when dispayed about 900 to 1000 pixels wide.
In the comparison test at the Apple store, we were standing directly in front of the regular and 5K iMacs. In order to see the subtle differences, we were looking from a distance of about two feet, and certainly no farther away than three feet. Since I had already ordered a fully loaded 5K iMac, I was eager to justify my purchase and order. I looked at the Macs at a closer distance than I do at home. The store manager was even more eager to point to any differences, and so in one case where a high resolution image appeared to have about the same level of detail on both Macs, she pointed to a slight difference in color, which I ignored, for reasons that we are aware of. But in that case, in another large MB high resolution image, there simply wasn't much of a difference in terms of the detail.
Regarding the web fonts, I entirely agree. My point in that case was that one of the Apple sales people had to blow up a web page to a huge degree, probably close to 700% to 1,000%, in order to demonstrate any visible difference. That was his test to "prove" the difference, not mine.
I didn't look at regular fonts and they certainly might look better on the retina display, just as they do on a Macbook retina display. Personally, I wouldn't spend $3,000 to $4,000 for that slight improvement in fonts, but an editor who looks at type all day long might do so. While I type this, I am aware that some smaller web fonts are slightly blurred, especially the smallest fonts. I assume a $3500 5K iMac could improve on that.
There is no question that there is a difference, and I saw that on a few 300+ MB high resolution images, while I could not see a difference on others. And I assume the same would be true of fonts. IMHO it is a subtle difference, but others might disagree and call it a significant enough difference to justify the purchase. It is subjective and in the end, a personal decision.
It should be noted that others in this thread said that they would wait for future 4K or 5K displays that would perform better than the Apple retina display regarding color rendition, etc. That might be possible in the not too distant future, in light of the following report on improvements in support for third party 4K displays:
http://www.anandtech.com/show/7847/improving-the-state-of-4k-display-support-under-os-x
