Mentioning APL in this context makes me picture an 8 bit home computer with an APL interpreter. I mean, the C64 had all kinds of weird symbols on its keyboard, too. And the magazines would save quite some pages for their source code listing compared to BASIC...
For Forth, that's an easy answer: when you think "hacker" you're thinking of people working on the Big Iron. Forth's niche was small machines, so while it had a good bit of popularity on the micros, its influence on the mainframe hackers, with their "we hate micros" ethos, was minimal.
I suspect younger programmers can't even begin to imagine what it was like. I learned to program on the Commodore 64. The default language was (Microsoft) Basic. Line numbers. Only flow control IF, GOTO, and GOSUB/RETURN. No loops, no ability to write functions, nothing like a define or a macro.
And then I got my hands on a Forth. Functions. Control structures. Maybe they weren't fancy by modern standards, but they were easily an order of magnitude better than Basic's, and you had the ability to write new ones fairly easily. Quick compiles, code which was both small and fast. Easy inline assembly code for when you needed even more speed. It was a dream come true.
We younger programmers started with Scratch. Sure, it makes pretty pictures, and you don't have to type, but it has only global and object-local variables, barely has function calls, and is generally awkward.
It wasn't until the 2nd or 3rd time I'd used it that I actually figured out how to make sense of it and run something (for Scratch's definition of "run").
To be honest I've progressed extremely slowly with CompSci/programming over the past 18 years I've been using them (got my first computer around 7-8) - I started with QBasic, been shouting at PHP for way too long, I have a basic understanding of C I badly need to develop, and I'm moving toward playing with Lua next - and I hardly consider myself a dyed-in-the-algorithms academic type with a brain that's unable to understand Scratch. (In fact, I'd argue that the best programming teachers would be precisely those types of people, and if they were unable to understand Scratch that would be a major problem.)
Rather, I firmly believe Scatch's UI is a disaster, and horribly unintuitive to use. Other languages are beset with grammatical idiosyncrasies; with Scratch you have to learn the UI before you can learn the... few parts of the language that are actually there.
I'm concerned that systems like Scratch are so widely used; I fear that it's an even worse mind-scrambler than the bad sides of BASIC. Of course, like BASIC, there are good sides, and it teaches the basics without presenting a Mt. Everest-sized learning curve. Perhaps https://en.wikipedia.org/wiki/Dartmouth_BASIC was the Scratch of 1964, and I'm just griping about the dilutory effects of educationally-targeted software in this day and age and "modern" GUI design.
Scratch is also really slow/laggy on my old laptop (Thinkpad T43), I can't imagine how bad it is for schools with limited hardware.
When I used Scratch 1.4, I don't recall the UI being too bad, and it ran pretty fast: Smalltalk is pretty good at that. But I'm generally pretty good at picking up these sorts of things, and my computer wasn't particularly slow.
If you want a Real Language presented the same way, Snap! (descended from BYOB) is essentially a Scheme in Scratch's clothing.
But the two real draws of Scratch were its hackability and its community. Back before Scratch 2.0 ruined everything, Scratch was written in Smalltalk, and using a widely-known hidden feature, you could examine the source code and make whatever changes you wanted with relative ease, resulting in a healthy community of mods and derivatives which explored new features and ideas, or those that the official team had dropped by the wayside (like Mesh, a fully-featured networking system).
Scratch's community was likewise excellent: I spent a lot of time lurking in the Scratch Advanced Topics forum - a sort of off-topic general programming section, where people far smarter than I discussed modifying Scratch, improving the website, and whatever programming projects they happened to be working on (usually web programming in PHP - it was the mid 2000s, after all).
Ah, I encountered Scatch 2.0. I'll definitely check 1.4 out, it looks a lot more accessible and reasonable. I would have loved to have encountered something like this at 14 or 15.
I suspect the reason Scratch felt slow to me is that 2.0 is some kind of HTML5 and/or Flash mess now - you're right, Smalltalk is really fast. I spun up Squeak to check something on this T43 yesterday, and everything was really snappy. I have no reason to expect Scratch will be any slower.
Also, I wouldn't be surprised if a reasonable bit of the exploration everyone did was motivated by the fact that they were "hacking" the platform :P
There seems to be a sad lack of excellent online communities nowadays; I've long looked for sites to complement HN, but without success.
I just had a look at Snap! which is interesting. It definitely flatlines this laptop though, I had to try it on a faster machine. But I'm running the tree animation demo right now, and it looks awesome....
For what it's worth, after a bit of research [1], I discovered Commodore Basic 2.0 had simple FOR loops (which I think I used and forgot), and the ability to create functions which encode a single one-variable mathematical expression (which I don't think I ever knew about).
I forget what the name was, but I used a Microsoft Basic on the IBM PC platform a few years later which was light years better than the C64 Basic. Still gave it up ASAP after learning C, mind you.
Forth was quite popular in Europe thanks to Jupiter Ace and ZX Spectrum extensions.
https://en.wikipedia.org/wiki/Jupiter_Ace
http://www.worldofspectrum.org/infoseekid.cgi?id=0008717