I'd like to comment on your statement on the header compression dictionary. Firstly, your statement is only true for the initial header compression dictionary. Most of the improvement is achieved strictly through the use of compression at all, with very marginal gains from different compression algorithms or better initial dictionaries. If your implication is that intimate knowledge of HTTP is required for SPDY compression to work well, that is false. Please note the research at http://www.eecis.udel.edu/~amer/PEL/poc/pdf/SPDY-Fan.pdf with the following conclusion: "This result suggests that zlib’s adaptive dictionary evolves to roughly an equivalent state after compressing the first header regardless of the content of the initial dictionary."
Can you clarify what you mean about features not being added within HTTP itself? Where are you drawing the line for "HTTP itself"? I'm curious, since the ideas behind SPDY have already been proposed for HTTP/2.0, and indeed the starting point for the HTTP/2.0 proposal used the SPDY draft, so I don't know what it means that important features aren't being added to HTTP itself.