Ada is a language that had a lot of useful features much earlier than any of the languages that are popular today, and some of those features are still missing from the languages easily available today.
In the beginning Ada has been criticized mainly for 2 reasons, it was claimed that it is too complex and it was criticized for being too verbose.
Today, the criticism about complexity seems naive, because many later languages have become much more complex than Ada, in many cases because they have started as simpler languages to which extra features have been added later, and because the need for such features had not been anticipated during the initial language design, adding them later was difficult, increasing the complexity of the updated language.
The criticism about verbosity is correct, but it could easily be solved by preserving the abstract Ada syntax and just replacing many tokens with less verbose symbols. This can easily be done with a source preprocessor, but this is avoided in most places, because then the source programs have a non-standard appearance.
It would have been good if the Ada standard had been updated to specify a standardized abbreviated syntax besides the classic syntax. This would not have been unusual, because several old languages have specified abbreviated and non-abbreviated syntactic alternatives, including languages like IBM PL/I or ALGOL 68. Even the language C had a more verbose syntactic alternative (with trigraphs), which has almost never been used, but nonetheless all C compilers had to support both the standard syntax and its trigraph alternative.
However, the real defect of Ada has been neither complexity nor verbosity, but expensive compilers and software tools, which have ensured its replacement by the free C/C++.
The so-called complexity of Ada has always been mitigated by the fact that besides its reference specification document, Ada always had a design rationale document accompanying the language specification. The rationale explained the reasons for the choices made when designing the language.
Such a rationale document would have been extremely useful for many other programming languages, which frequently include some obscure features whose purpose is not obvious, or which look like mistakes, even if sometimes there are good reasons for their existence.
When Ada was introduced, it was marketed as a language similar to Pascal. The reason is that at that time Pascal had become the language most frequently used for teaching programming in universities.
Fortunately the resemblances between Ada and Pascal are only superficial. In reality the Ada syntax and semantics are much more similar to earlier languages like ALGOL 68 and Xerox Mesa, which were languages far superior to Pascal.
The parent article mentions that Ada includes in the language specification the handling of concurrent tasks, instead of delegating such things to a system library (task = term used by IBM since 1964 for what now is normally called "thread", a term first used in 1966 in some Multics documents and popularized much later by the Mach operating system).
However, I do not believe that this is a valuable feature of Ada. You can indeed build any concurrent applications around the Ada mechanism of task "rendez-vous", but I think that this concept is a little too high-level.
It incorporates 2 lower level actions, and for the highest efficiency in implementations sometimes it may be necessary to have access to the lowest level actions. This means that sometimes using a system library for implementing the communication between concurrent threads may provide higher performance than the built-in Ada concurrency primitives.