- He hasn't shipped anything popular (he's a researcher, expanding the tree of knowledge, not an engineer optimizing for shipping) - Smalltalk isn't as great as Alan makes it out to be (again he's a researcher) - That the computer revolution has happened but he's stuck in the 70's - Many, many more things
Just because we do something doesn't make a better idea than ideas we had in the past that are not being used. There are lots of dead great ideas.
And honestly, the language itself isn’t that remarkable by today’s standards; there’s so many dialects floating around most of the features have made their way into mainstream languages. It’s the tooling that I dream of. Sadly, it’s probably also another reason why it never took off: people couldn’t easily incrementally adopt it.
In my little corner of the industry, we've adopted a big heap of things for entirely political reasons, and they're mostly crap in one way or another.
At the end of the day, languages that were less extreme won the popularity contest. Ruby and Python, for example, are just more accessible and full-featured.
Smalltalk or more specifically one of it's dialects, Self (an even more "extreme" Smalltalk), made huge advances in JIT compilation. Advances which later ended up in the Java HotSpot virtual machine. Some benchmarks hit half the performance of optimized C. That was in the late 90s.
I will google these questions also, but am always interested the HN crowd's take. Like the original question I didn't know he'd ruffled any feathers.
Edit: top hit was Alan Kay himself addressing criticism on a Quora question: https://www.quora.com/Has-anyone-criticized-Alan-Kay, but on further read it obliquely addresses criticisms other than his role in developing OOP.