OK, judging by the graticule on that version of the map, it's Equidistant Cylindrical, with standard parallels at 42.4 degrees.

That is not a Mercator graticule so the distortion behaves differently. In Equidistant things get stretched out east-west (horizontally) as you near the poles, but distances remain constant north-south (vertically). Mercator compensates for this by adding a north-south stretch equal to the east-west stretch. So everything gets bigger as you near the poles, but it does so consistently and so it preserves angles. (in cartography speak, it's "Conformal"). If that doesn't make sense, think of it as preserving the shapes of things. That's not exactly correct, but it's the easiest way of thinking about it.

Mercator has a rather odd property. It's stretching each parallel out to the width of the map, and then adding the same stretch to the height. The poles are just single points though, so in a rough sense, stretching them out out to lines is infinite stretch, so there needs to be infinite vertical stretch, so the poles are infinitely far away from the equator. In practice, you have to decide how much you want to cut off at the poles.

If you want your map to be in the Mercator projection, you need to use a different graticule. The spacing of the parallels (horizontal lines) needs to increase as you move away from the equator. you can get a Mercator graticule here: http://www.cartographersguild.com/ma...-template.html It spans from 85 S to 85 N. But you can trim off the top and bottom if you want.

On the other hand, it's the only projection which lets you put a compass rose or straight rumb lines (the lines radiating from compasses on some maps) on a global map. All other projections distort compass directions.

Equidistant Cylindrical is generally easier to transform. For instance there's a handy little program called G.Projector that can take equidistant maps and project into just about anything you want, and you can easily wrap it around a globe in 3D graphics software. The difficulty is that it's hard to draw in the first place because of the uneven distortion, and you have to reproject it into something else to get a decent map.

Mercator is easier to draw because the distortion is consistent. You still need to be aware of it, but it's easier. You can also use it "as is" for a world map, and you can get away with zooming in on it to get regional maps, although it's still problematic. The downside is that there aren't any simple tools for re-projecting it like G.Projector. You need to go through a more complicated process and use more advanced tools like QuantumGIS. It's not actually that great a choice for a general reference map, but it's a bad choice that was popular, and it looks a lot better than Equidistant Cylindrical.