some guys like some extra resistance, and given the same tip opening, some guys don't like any resistance that's not part of the normal tip opening/reed strength/facing length combination and want all other resistance cleared out. Some of the things that usually cause "bad" resistance are:
Uneven rails anywhere on the facing
An improperly balanced curve (i.e. a curve which has bumps or flat spots in, or that just generally doesn't math up with a smooth curve-type
A convex table
A table that is too concave can feel funny to play
A window that is too short (doesn't extend far back enough into the chamber)
A bulge of material underneath the table "U" shape in the chamber
A large wedge-type baffle that extends too high into the chamber
A tip rail that is too thick
A curve that's not balanced in proportion to the tip opening and facing length
A facing length that is too short
There's probably more stuff, that's just some of what's coming into mind at the moment...
The biggest thing I can think of that adds what some people consider "good" resistance is adding what's known as a more elliptical shape to the curve. This means setting up the facing curve so that, given the same tip opening and facing length, the curve is a little wider, more like an oval than a circle. This makes the reed have to bend a little bit more to close off at the same place as it does with a radial curve. As long as the curve is based on a mathematically sound set of data, and it's still a smooth curve with no bumps or flat spots, an elliptical curve just adds a little extra "good" resistance to a mouthpiece.
You can also choose not to change some of the things listed in the first set of "bad resistance" things, which are not always desirable to change (it depends on the player's needs and how the mouthpiece is currently responding), or you can only change them a little, etc., to achieve the desired result.
I personally kind of think that resistance should be managed mostly with tip opening size and reed strength, and a facing length should be picked according to the strength of reed you play and the tip opening you choose. With less resistant set-ups like a small tip and a softer reed, the shorter facing may work well, medium tips/reed like a medium facing, and more open tips and medium-hard to hard reeds will work better with a longer facing. Some players do like the elliptical facing for their set-ups, possibly because they have naturally stronger playing chops or a stronger diaphragm or both, but I usually like the radial (regular or circular) facings because they allow for the most instantaneous attack and free-blowing feel. A person's opinion changes over time, though, so who knows, maybe I'll think something different next month... But for now, I think that removing all the issues that make a mouthpiece inefficient (the problems listed above) and making the curve a radial, free-blowing-type will yield the biggest sound and most easy-to-play mpc for a given tip opening. That way, you can maximize the tip opening to where you feel the most comfortable and also get the most out of it by picking a reed strength and facing length that are a good match for it without having to fight against those other properties that are found in many factory-made mpcs that are detrimental to a mpc's efficiency.
Just the other day I opened up the tip on a Rousseau alto mpc I had sitting around for kicks to see what it played like, and in the process of refacing it I flattened the table. The table on this mpc was so uneven and warped that it was silly. It's too bad that, if they are going to use machines to cut the facings on a lot of the factory-made mpcs out there today, they don't get some machines that work a little bit better! Ha... I mean I'm just assuming this was a machine-cut piece, but seriously, the table was warped and the whole facing was twisted badly... I think the left facing length gauge was hitting at like 34, and the right side was hitting at about 44 on the gauge! It was twisted that way all the way up the curve... Definitely fell into the mpcs-with-bad-restance category!