Check the charge threads referenced in the FAQ. Different people have different experience with charge, and double click works perfectly for me if the conditions are right. Single click basically means the unit will be less likely to lose cohesion due to various other factors, and therefore more likely to do a formed charge.
Smith is correct, CA has stated on the blog what the new charge mechanics are. Double clicking will result in the charge stopping when 10% of something occurs, I forget if it is impact or casualties. Single clicking does not have this limitation. It's not to say that one can't double click and still get an effective charge, it will just not be AS effective as a single click.

It should also be pointed out that CA toned cavalry down a bit and made it harder to get those massed charges, terrain unit cohesian, moving targets all have an impact. basically they were trying to avoid the RTW style battles with the massed cav charge wiping everything in its path away. Cav are still powerful when used well and properly they just are harder to use and take some more getting used too.
IIRC these changes were largely in part due to the multiplayer community's complaints about cav spamming. I don't necessarily mind the new charge mechanics, but the unit cohesion from RTW to M2TW to a steep turn for the worse. Keeping my army in formation when moving normal or doubletime is just about impossible, and it's not due to lack of skill either on my part. Still why I (and many others) say that the MP and SP mechanics and stats should be separated, so the MP folks can get the balanced game they want, and the SP types can still have our powerful heavy cav charges like they should be.