The following seems to be true: if $|W_q| := \sum {q^{l(w)}}$, where the sum is taken over the elements $w$, then $|W_q| = \prod {(1 + q +...+ q^{e_i})}$, where the product is taken over the exponents $e_i$.

In other words, if $V$ is the root space for $W$ and the polynomials $f_k$ are the basis of $S[V]$ sur $S^W[V]$, then the set of $deg(f_k)$ is the same with $l(w)$.

I couldn't find any proof for this. Yes, I tried Bourbaki.