I believe the "everything is a ___" mindset causes problems, regardless of what fills in the blank. In TCL, everything is a string. In LISP, everything is a list. In Smalltalk, everything is an object. The fact of the matter is not everything in the world is homogeneous. Some things are objects, others are numbers, and yet others are actions. Forcing everything into the same conceptual model forces the programmer to perform cognitive acrobatics.
Understanding the semantics behind a simple statement such as Transcript show: 'Hello World!' is easy; a Transcript window object is sent the message show:, which then responds by displaying the argument. But the semantics behind conditional code execution isn't so intuitive. Smalltalk doesn't have a dedicated if-construct, instead conditional execution is achieved by passing messages to Boolean objects, like so:
x > 0 ifTrue: [Transcript show: 'Positive'] ifFalse: [Transcript show 'Negative'].The object represented by the variable x is sent the > message with the argument 0. x then compares its value with that of the argument and returns a Boolean object in response. If the value of x is a positive number then the resulting truth value of the returned object is true, otherwise the value is false. The ifTrue:ifFalse: message is sent to the implicit Boolean object providing two code block (which are also objects!) as arguments. All basic language constructs such as if-statements and while loops are abstracted as objects! While it does produce an appreciable amount of elegance and flexibility in the language, this thinking takes some getting used to.
There's a lot going on behind the scenes with an expression like 2 + 1 / 4 as well. A language like C will follow the rules of precedence stated by the mathematical order of operations. First 1 is divided by 4 which yields .25, and then 2 is added to .25 resulting in the final value 2.25. In Smalltalk, everything is an object which the programmer interacts with by sending messages. The literals 2, 1, and 4 are actually instances of SmallInteger objects. The + message is sent to 2 with an argument of 1, which responds by adding the value of the argument to its own value resulting in a value of 3. Then, the / message with the argument 4 is sent. The object responds by dividing its value by 4 which results in a final value of .75 for the expression.
Some things can easily be conceptualized as objects, such as windows, streams, and data structures, while it can be difficult to think of others as objects, such as text, integers, colors, and truth values. Over-personification and the perspective of the grammatical predicate/direct object performing actions is backwards of how we speak and how we think.
As awkward as the "everything is a" mindset it, I have to respect Kay for it. He intentionally adopted it so he and his team would be forced to think outside the box. Kay admits "we were actually trying for a qualitative shift in belief structures-- a new Kuhnian paradigm in the same spirit as the invention of the printing press-- and thus took highly extreme positions which almost forced these new styles to be invented." I can respect his effort and appreciate the elegant design it resulted in, but I can't expect an 8-year old to understand passing code blocks as arguments and why the order of operations is different on the computer than in the classroom.