Seriously, why is this not more common? When I go to your landing page I want to see how awesome your thing is. A few examples is all it takes.
Show me something like this
In[6]:= Derivative[1][Sin]
Out[6]= Cos[#1]&
In[7]:= Derivative[2][Sin]
Out[7]=-Sin[#1]&
Don't make me go to documentation. Also don't compare it to Mathematica if it isn't as intuitive as Mathematica (seriously, it is wildly popular because simple things you don't have to look up. It works like you expect. There was a lot of planning to make it this way. People note the "Zen" of Mathematica). Here's an example of where Mathics is non-intuitive (besides if you remove the [1] from the above example)
n[13]:= x = 5
Out[13]= 5
In[14]:= Derivative[1][x^2]
Out[14]= 25'
In[15]:= f[x] := x^2
In[16]:= Derivative[1][f]
Out[16]= 2 #1&
In[17]:= f'[x]
Out[17]= 10
I know no system is perfect, but try to sell me on your idea before I go through your documentation and get frustrated that it doesn't work like the thing you compare it to.