The general "wag" is that for a bike that will never be off a prepared surface you need 1", for a bike on moderate chop 2"-3", and a bike that will go full rock hopping 4". This is a safety thing. I fully agree with those who said that standover DOES matter. Not only that, it's the only dimension of your bike that you cannot change.

The reach will be affected to. For instance, my road bike has almost 2" of standover. My MTB has over 3.5". The reach to the flat part of the drop bars on my road bike is not too much greater than the reach to the bars on my MTB. (Of course the hoods and drops get me more stretched out than the MTB could). MTBs are geometrically constructed differently, and a bike that is too tall is probably way too long. If that is comfortable for you (and is should be on prepared surfaces) you may want to look into a road bike. On a trail being stretched out may not be so advantageous.

Like another poster alluded to, you can change other dimensions of a bike with longer stems, etc. But you cannot re-create the frame to give you clearance. The greater standover for a MTB is so if you need to put a foot down on irregular terrain, perhaps terrain in which part of the bike is resting on something a bit higher than the ground under your potential foot rest, you can do so whilst preserving your nether regions.