I see a few staff members and users on the forums that complain about what Snap! looks like (mainly the flat design). If the appearance of both the community site and the editor is not considered that great, why haven't they updated it?
I'm not sure why the concept of "outdated" applies to appearance issues. 100 years from now, for example, serif fonts will still be more readable than sans-serif (because you can tell the difference between a capital I and a lower case L, for example) despite how everyone says sans serif is "more modern." If anything, increasing screen resolution makes the readability that much greater, because the serifs can be weighted as the font designer intended.
I think the concept of "outdated" is applied to appearance issues because certain styles represent older internet eras. Many people define "modern" as flat, simple, and sleek. New company websites and web applications exemplify this.
I believe that Snap! looks outdated because it was modeled after Scratch 1.4, which was released in 2009. Websites and applications in 2009 were skeuomorphic and neumorphic, filled with shadows and highlights. The default Snap! editor reminds me of this time, so I relate it to "outdated" designs. Personally, I'd like to see a revamp. In my opinion, the editor should keep basic shadows, remove the highlights, add more text spacing, lessen the stroke size of the font, and remove decorative grid lines. I understand that this is only my opinion, but I think that it would simplify the design and provide better UI/UX.
Oh, I understand what the criteria are; I just don't see why they're important in a non-information context. I mentioned serif vs. non-serif because that affects the readability of actual information, but your example of the striped background in the scripting area doesn't. (It was a pain in the neck in the early days, before we had the script pic feature, so you had to take screenshots and then try to edit out the background, but now it's no problem.)
About shadows, bear in mind that in Snap! they bear information, e.g., distinguishing an upvar from a variable used as a regular input expression.
I confess that my feeling about this is affected by my general feeling that things keep getting worse and worse, which is why each new computer I get runs slower than the old one (because of software bloat). My take on "internet eras" is that the main turning point in its history was the disastrous invention of the World Wide Web, which changed the Internet from mainly a vehicle for personal expression to mainly a vehicle for corporate expression, and that happened in large part because people came to expect professional page design. (To be fair, the Web wasn't originally planned that way; the original idea was that servers serve information, and browsers are responsible for its layout and design, following the desires of the user. But they made the mistake of allowing servers to send formatting instructions, and that quickly made it mandatory for servers to do that, to meet users' new expectations. And today even (ADA-required) respect for minimum font size set by the user is largely ignored by many servers, which try to cram the text into their designer-designed tiny text boxes without scrollbars.)
Opinions expressed are my own, and do not reflect official positions of the University of California, etc etc.
Yes, brightness is its biggest bug. But I just don't like the flat shape. (This is a rare example in which I think professional design (in the regular design) gave a much better result than the less-designed, modern, "flat" version.)