Well, I can answer at least a few of those 'WHY?!'s.
Firstly starting with my_string['foo'] = 'bar', this is a syntax that is used a lot in Ruby for anytime that you wish to recover a subset of a collection - you can use it with a Dir object for example to recover a subset of directories/files, or in the case of a hash, to recover the object stored at the key. The thing is that you need to think about it as being a subset operator, rather than an index operator.
The exclamation point at the end of a method name has no syntactic meaning - it is simply a naming convention used by the standard libraries to indicate when the method mutates the object or not. Same same for method names ending with a question mark, it's just a convention to indicate that the response will be a boolean.
As for the whole "5" * 2 vs. 2 * "5", I must admit to not understanding where you're coming from. I don't see how you could expect any other kind of behaviour from an object orientated language. Personally I don't like over-riding arithmetic operators for non-number types, for precisely this reason, but hey, some people like it, so why not?
Integers converting to strings? I think every language has that no? I mean there's always something like:
sprintf(myStringPtr, "%d", myInteger);
or
cout etc etc
By making this a standard method on all classes (to_s), Ruby makes it really easy to dump a class to a serial format, which is very useful for debugging, as an example.
I get the impression that you simply didn't spend enough time with Ruby to come to understand its idioms, in which case, you're not really in a position to effectively criticise the language...