There was a 10x quest experience bonus in Shroud of the Avatar, but it has been disabled now. But the reason is actually somewhat mundane and disappointing because it all ties back into other systems being exploited. Ages ago, shared XP pools for multiple characters were added by a developer who, according to the post on the matter, did so without being told to and without consulting with anyone. Once the feature was in, it was hard to remove or adjust.
This was all well and good for a while, but with 10x experience from quests, players were making new characters, doing several quick one-time quests, then deleting the new character, making a new one, and repeating the process. The feature has finally been tracked down and hopefully closed off to avoid these exploits in the future, with the exploiters who used this trick getting punished accordingly, but it will hopefully result in a better overall experience and ultimately will lead to the 10x buff being turned back on. Bit of a catastrophe leading up to that, though.
