Good question mate.
If I recall correctly we have discussed this recently. The most common 'measure' is blade gap - but the head design is also critical IMHO in determining how aggressive a particular razor is - impacting the projection/exposure and angle.
I rate aggression by feel on the face and propensity to let me know if I make a mistake (the margin of error!). So with a R41 2011, a commonly described aggressive DE I can get a great, smooth, irritation free shave OR if I don't pay attention and go gentle a face full of nicks. Use it for a week though and my face says ENOUGH!
Sometimes I find an aggressive razor, eg R41, goes better with a sharper blade, eg Feather, than a more regular variety. While the R41 with Feather would generally be called 'more aggressive' I find using it results in less irritation and less nicks (maybe because more care is being taken) but, after a week or so, my face still says - respite please! I need to have two days with a milder combo.
Some SEs are similarly aggressive, MMOC, others less so, 1912 (?), but all with a different approach needed to DEs. The blade is stiffer, that angles are different - just a whole new experience!
I cannot comment on straights I'm afraid.
