So on to the second part. (This is the first one.)
When I started, I soon settled for a d6 pool system with 4+ as the standard success threshold. Where Donjon had used Attribute + Skill + Bonus Dice, I chose
+ Bonus Dice.
To explain this difference, I need to divulge a little bit about my gaming socialisation. In the old World of Darkness rule books, there were some hints for every level of a trait on how that particular level would look and feel in play. That's great. I'm all for this method. It doesn't work in WoD though, because you always use the sum of two traits. The information gained from the descriptions was meaningless.
Scarred by this experience, I decided that, in my games, every trait you write down would be used as is for at least one purpose. With this precept, adding attribute and skill wouldn't work for me, and besides I had other plans for my attributes.
Since players might want to use untrained skills, there should be some dice even before the skill level. After some experiments, I settled for 3d6.
I also decided to limit the number of bonus dice. Bonus dice can show special advantages, help from allies and preparation. That's nice, but I didn't want players to think about them for too long. First, I tried to use rules like "you cannot more than double your dice pool with bonus dice". That worked, but made characters who were good at certain actions even better. So finally, the limit became 3 + Character Level.
With bonuses accounted for, how would the game treat difficulties? There are three rather obvious ways to do it: Take away dice before the roll, take away successes after the roll, or change the success threshold to a higher number.
Last things first, fiddling with success threshold wasn't a good option. With a d6 and a basic threshold of 4+, there is only 5+ and 6 to go. This method works much better in Shadowrun, because Shadowrun employs exploding dice.
Remaining we have substracting dice and substracting successes. Substracting dice is used in nWoD, and it's pretty. Because after your roll, your result is the result. When you roll four successes on an attack, the opponent will take four damage. That makes the game feel quite brutal.
There is a disadvantage, though. You cannot possibly roll, before you know the difficulty. You cannot proclaim "I smite thee!" and roll the dice. You need to learn your opponents defense first. Since I was looking for a more heroic game, I didn't much care for nWoD's immediate effectiveness. So taking away successes it was.
 Note that nWoD doesn't have such information anymore.