- really nice strict typing (you can define a type which represents an number which varies between two values which are not known at compile time and the compiler will enforce it. And then you can define an array with it as an index, and it'll just work).
- fully nested everything with seamless upvalues. First class support for nested functions makes a lot of things easier.
- amazingly nice, rendezvous-based, type-safe message-passing concurrency. Think Go channels, but better.
- full support for generics. (e.g. the standard complex number library is supplied as a generic package so you get to specify what number type you want it to use.) There's a standard genericised container library.
- sanitised pointers. It's actually syntactically invalid to leak a pointer from an inner scope to an outer one.
- full OO support (although it's been shoehorned a little uncomfortably into the syntax; they should have added some more keywords).
- full low-level control over structure layout. You can specify the meaning of every bit of a structure, if you want (including, IIRC, endianness). If you want to.
- built-in support for design by contract. Most things you can specify preconditions and postconditions, and the compiler will enforce them.
- really fast. Like, about the same as a good C++ compiler.
The tl;dr is: fully compiled, old school systems language with rather antiquated syntax but an amazing feature set.
I did a writeup last year: http://cowlark.com/2014-04-27-ada
...and interested parties might like to see a multithreaded Mandelbrot what I wrote: http://ideone.com/a1ky4l