Those sound like good ideas, I'll be interested in hearing what knowledgeable people answer. The pulling out of his queen's bishop can sure signal a promenade to Qb3 by your queen.
From bitter embarrassing experience I've come to conclude it's basically never a good idea to try to put the Queen in front of the bishop to hit h7 in this type of opening, maybe we can call this "the parham technique". 
If he puts a bishop on the 4th or 5th and it's ungaurded, always look out for tactics with the queen.
After you are well developed and some pieces are gone, you may be fully justified to bringing your queen out to f3 or sometimes it might even go to h5. Like the Qe1-g3 queen manouvre in many sicilians.
I sometimes find myself wondering where to place my queen with White in d4 openings.
Are there any good general rules or abstractions that could help me here?
My current approach is this:
Put it on c2 most of the time. It supports the c4 pawn (especially in Catalan this can be important) and can support an eventual e4 push. Put it on d2 if I have a bishop on the c1-h6 diagonal and I want to exchange his counterpart on g7. Put it on b3 if Black developed the light-squared bishop early so I have some pressure on b7 and maybe d5. a4 only if I desperately need a check to recapture in c4 or need an extra attacker after putting pressure on a c6 knight with Bb5.
Of course there are always concrete exceptions, but even from my general scheme I sense that there could be improvement.
Which piece exchanges favour which square (for example the BxNf6 exchange, one an exchange of a pair of equivalent minor pieces)?
I know my question is very general and might be hard to answer, I'd still appreciate some advice.