const by default may not be a bad idea, but immutability of structures/objects in C++ sounds like it would be a huge performance hit to me. I've used an image processing library (CIMG) which lets you do things like `img = img1 * 2 + img2 * 0.5`, instead of editing the image in-place. It's pretty, but with regular 800x600 images the performance was around 2 orders of magnitude below a simple for loop... it made a lot of extra copying and allocation instead of mutating the image.
The thing is, this kind of code isn't an exception in the C++ world. C++ is made for problems where performance matters: image processing, compilers, browsers... immutability works for Clojure, but Clojure is a lot slower than C++. It's more of an "enterprise software" language.
In retrospect, it might have been better to have a keyword for things that can change. This goes equally for Java and "final". Making immutability the default and mutability opt-in makes for fewer bugs and more reliable programs.
Arguably, this is what Clojure does where you have functional, immutable constructs by default with ways to declare things that can change (refs, etc.).