What's Great about Dice-Pools
What's great about dice-pools is meaningful results, all the time. You'll struggle to get that with a D20.
Listen...
Example 1: Good Chances
The party ranger wants to navigate through the forest. Forests are dangerous, and he might fail, but his bonuses mean he succeeds on the roll of 3 or more.
A 90% chance of success makes the roll a little less interesting, unless the party stand a lot to lose. Even then, this roll will produce the result of 'business as usual' 90% of the time.
The dice-pool mechanic works differently. Let's assume White Wolf's D10 system is in play.
- successes: the party are waylaid, and lost.
- success: the party make their destination after a day of detours.
- successes: the party make their destination in four days - not great, not bad.
- successes: the party arrive after three days, as expected.
- successes: the ranger notes a particular area floods this time of year, so the party can travel half the distance by boat, and make it in two days.
- successes: the ranger organises the boat-trip, while finding a short-cut, and waking everyone early. The trip takes one and a half days.
If we assume that same ranger has a dice-pool of seven, we can be pretty sure they succeed in their roll. But now we know how well they succeed, and the result becomes interesting again.
Example 2: Bad Chances
The wagon's wheel has fallen off, and the floorboards are rotten. None of them understand woodwork very well, so they succeed on the roll of 18 or more.
A 15% chance of success sounds pretty bad - we can be fairly sure they won't make it.
But the dice-pool mechanic works differently. The best person at crafting might only have a dice-pool of 2, and they might need to roll 9+ to succeed, but they still stand a reasonable chance of something happening.
- successes: the cart does not work, as expected.
- success: they can get the cart moving enough to get it to the next village, with someone holding the wheels on whenever it turns left.
- successes: the cart will go for a few miles before breaking down again.
None of these results are really a 'success', and the top result isn't very likely. But just having two different types of failure once again makes the roll interesting.
Example 3: Sideways Chances
The bandits chase you onto the icy lake. Everyone suddenly feels a little less sure of their footing.
In a D20 system, you might get a penalty for ice, and so do the enemy. Not much has changed, since you both get the same penalty.
But (and I'm sure you can see what's coming) the dice-pool allows you to shift the number you need to roll to successfully attack (or talk, or plot, or anything else). That affects consistency.
If the dice start by succeeding on the roll of 4+, then the results will match the ability scores - the person with the most dice wins. But if the difficulty shifts, and the dice succeed on 9+, then the results become less predictable. The weaker can defeat the stronger when the situation becomes unstable.