I could make small arguments on both sides of all of these that wouldn't really change the dynamic of what you're saying a ton, but #4 is huge.
GDDR5 is to Ferrari as DDR3 is to Ford Escort.
That being said, it matters what devs do with it. Like you said, exclusive first party titles will shine, and though multiplats may be similar early in the gen, I'd argue that the gap would widen as the gen evolves. A few points:
-PS4 is more developer friendly than PS3, moving from Cell to x86
-The gap between DDR3 and GDDR5 is pretty wide
-It is rumored that the PS4 OS will take much less memory to run than that of the XBox One.
If the PS4 becomes the multiplat master platform with more, faster memory and XBox One becomes the ported console, look out.
Personally, I think Microsoft can overcome most everything else to be competitive this gen. This is the one that they're not going to be able to do much to compete with.
