So a list must measure itself against the three phases of the game: movement, shooting, assault. A list needs to be strong in two of the three areas and at least able to hold its own in the third. Remember this is a relative ranking, so Razorspam vs Razorspam Marines aren't "strong" against each other for movement, whereas most DE builds are always strong in movement vs any Marine list, however a mechanized Marine list can hold its own if it is superior in shooting and assault.
NOTE: Remember that shooting & assault can both change these relative scores. If I just killed every vehicle in your list in the first couple of turns, then your movement is now weaker. Similarly if I just shot your rock CC unit to pieces, then it isn't a big CC threat anymore.
Now this does not mean you have to have fast vehicles for every unit. It just means you cannot be weak and stay weak in movement. My Deep Strike list lives to de-mech my opponents to put them on foot. I eliminate their speed advantage by putting them on foot like I am as quickly as possible.
A list on the "razor's edge" is one that is weak somewhere and cannot hide that weakness until it uses its strengths to compensate for its weaknes