[EdSchouten]

The idea of this encoding is to store two bytes of data in three characters. To me it's not obvious why you need a base as high as 45 for that.

Assuming you either want to store two bytes, or a trailing one, you have 256*256 + 256 combinations: 65792. Using three base45 characters, you can get up to 45^3=91125 combinations. It looks like base41 would have been sufficient. That way you can get rid of some of those special characters, making it easier to use through different transports.

[dolmen]

+1

This would allow to avoid space, %, / and + to be more URL friendly, and so allow more usages.

[kristov]

Having these chars is a pain (particularly space). Manipulating lists of these in files with standard Unix tools will mean having to escape, quote, etc.

[mr_mitm]

I don't understand why they didn't use lower case letters instead of special characters. Does anyone here know?

[lifthrasiir]

You can't put lowercase letters in the alphanumeric mode (5.5 bits per character) and need to switch to less efficient byte mode (8 bits per character).