Normal SL: Tens of your skill minus the tens of your roll. So if you have a skill of 54 and roll 12, your SL is 5-1= 4.
Fast SL: On a success, SL is the tens of your roll. So if you have a skill of 54 and roll 12, your SL is 1. On a failure, as normal.
One requires subtraction which takes time, the other just checking the die result. From an aesthetic perspective, one might prefer that SL be connected to the margin of success, but the probabilities are similar if not identical (fast gives you more SL 0, normal gives you more maxed SL).
Note that Fast SL alters the odds of SL=0 up, and max SL down.
That difference in the underling math is why it gets so broken if you try to play without the Fast SL rules. The system was designed to use Fast SL for everything- the standard SL was added in after the playtesting was done.
Not that going Fast SL will fix everything, or all the other issues I have with the game.
The REAL problem is 4e is too dang complex in comparison.
And this is the real kicker for me IMHO. Others have given the laundry list of problems.
Fundamentally there is just no need for this much added complexity.
There was nothing about 2e that a solid round or two of playtesting couldn't fix.
Yes spellcasting could have been brought more in line with how 1e worked. That would be fine. And again plenty of longtime players already had homebrew fixes. That experience base could easily have been tapped during playtesting.
It just irks me that all the time was wasted taking something that just needed to be refined, and instead doing a blank slate design which "solves issues", yet introduces as many new problems as it solves due to the increased complexity.
C7:
"Hey we got the license for WFRP - 4e is coming!"
WFRP Fans:
"Fantastic, a refinement of what 2e did would really be great!"
C7:
"2e WFRP!? Pfffttt.... No no no. We are bringing WFRP forward to a new era of rules complexity. Gotta bring in all those new fans!"
WFRP Fans:
"Dude what!?"
I think the problem, is when you write the system off as entire bad and a complete failure, at the same time as there are lots of people enjoying the game and it seems to be really popular then I have to question whether your measure of bad is the same as other people.
As a counterpoint.
In my opinion:
If people love the game world setting IP, they will put up with some downright broken mechanics to play.
Vampire 1e, Shadowrun etc, the list goes on.
My opinion continues to be that WFRP is a great RPG setting perpetually held back by mediocre systems.
4e just reinforced this view. It is yet another example of the game designers making the type of game that they want for their own table, without considering what would be best for WFRP
as a game.
4e is not quite as bad of as own goal as Seventh Sea 2e, but it certainly didn't do itself any favors...