I don't think page counts of specs or feature lists really tell you that much. I didn't find it that hard too get up to speed in Scala but it seems to scare away too many Java people.
My main criticism is that it allows too much syntactic flexibility.
I'm a programmer that in general prefers dynamic languages. Recently I've started writing production code in Scala for a couple of web services for which I really needed the performance and flexibility of the JVM.
Scala does have problems. But NOT the language. I find the language to be extremely elegant and well designed.
The problem lies with the community. I find Scala libraries to be an abomination of taste and common-sense. I don't know why that is, but in general I stay away from libraries commonly used by Scala developers.
For instance I prefer JUnit over ScalaTest, I prefer JAX-RS (DropWizard) over Scalatra or Play. I prefer Maven over SBT.
Of course, you could say that the language itself invites this nasty style of programming, because the syntax is too flexible and the features too powerful. However I strongly disagree.
For instance I've worked with a lot of Ruby and Python libraries over the years and most popular Ruby/Python libraries are extremely well designed, easy to use and easy to look under the hood. Ruby on Rails for instance was not pretty, however starting with version 3 it went through a major refactoring effort and now the codebase is clean and easy to follow; while being one of the easiest to use and full-featured web frameworks ever built.
Unfortunately when picking a language, you do have to rely on a community and an ecosystem of libraries. However in the case of Scala, if you don't like the style of the current community, you can just pick from the thousands of already available and mature Java libraries.
David Pollak has long been one of Scala's greatest champions but his grudging acceptance that Scala is hard, from a lot of experience in the field, is worth a read:
Yeah, but Scala is hard in the same way that Ruby is hard.
You can use a subset that's easier, however the usage of the more advanced language features need a level of understanding that's beyond the capabilities of many developers.
And this is in fact true of most mainstream languages. Java may be an easy language to learn, but Java isn't just a language, but a platform - and as soon as beginners start messing around with multi-threading (since multi-threading capabilities in Java are in your face), then all hell breaks loose ... from this point of view, Scala may in fact be easier to deal with than Java is for beginners.
And for instance I see people complaining about the method signatures exposed by the collections API. I can't read those signatures very well myself, however in the case of a dynamic language, such as Ruby, you'd have to look at the source-code to see what the method actually returns. So IMHO, even if those signatures are complicated, at least you've got signatures to look at.
Scala requires a more disciplined mindset. David Pollak actually developed Lift in response to problems he had managing big Rails projects so I don't think he would be advancing this analysis if he didn't think Scala was harder than Python/Ruby.
Yes, I'm full aware of that. I'm not claiming that it tells us the complete and final truth, just that it is an interesting data point.
In my experience a lot of those "I programmed 15 years in Java, get off my lawn" senior developers get angry about Scala. This is interesting, because it isn't that way for other languages running on the JVM like Clojure, JRuby or Groovy.
It is only a speculation why it is like that, but in my opinion it is because alternatives like Groovy, JRuby, Clojure are considered to be some sort of "supplemental" or "add-on" languages by those people, while Scala is seen as full-scale alternative to Java (not in the sense that it will replace Java, but in the sense of "I can write 100% of my application in this language without dropping down to Java for the performance critical parts").
I think it is interesting how different the reaction is compared to C# <> F#, for instance. Is it because Java and C# developers come from different backgrounds? Because F# is "made by Microsoft", while Scala is not "made by Oracle"? Is it because Java developers were happy with Sun telling them that they wouldn't need all those "fancy" features of the .NET languages? I think this would be interesting to discuss further, although I think it is hard to come up with valid data points.
From my experience, most of the claims about "complexity" and "too hard for beginners" come not from people learning the language, but from people with > 5 years of Java experience _not_ wanting to learn another language.
I'm quite ambivalent about the syntactic flexibility. On the one hand, they cut it down for 2.10, but on the other I think it is a superficial measurement. There are a lot of languages with more flexibility and a lot of languages with less, both seem to be alive and well. (Just want to make it clear that I think your stance is totally valid, even if I disagree slightly. Different people have different tastes, and this is a good thing!)
My main criticism is that it allows too much syntactic flexibility.