If you've read any of the community feedback on shadow priests, you'll know one of the biggest issues for the class is the poor scaling of mind flay — that is, as your spell damage bonus grows, mind flay benefits from a smaller percentage of it than other spells.
There are two likely culprits for this situation. One is the snare component of the spell. Other spells with snare components are similarly penalised. However, never by as much as mind flay. And that's not accounting for the spell's reduced range which is, in itself, probably due to the snare.
So while this might be a factor, I think the second candidate is more likely to be the main cause of the spell's poor scaling.
And that's the mana returned by vampiric touch. If shadow priests were allowed to scale as efficiently as other dps, the mana returned would be silly. Even as it stands now, it's considered too generous. That's the whole reason it's getting nerfed. So I suspect shadow priests have been hamstrung by the need to cap the effective amount of mana they have provided to their parties.
In retrospect, this was always going to be an issue. Mana pools don't grow significantly as a player's gear improves, so as spell damage and performance grew the mana returned was always going to be disproportionately significant.
Even with the nerf, this will remain an issue in Northrend, even if its effects will be less pronounced and targeted more at the under-geared player.
Blizzard clearly knows this, as evidenced by the two new mana batteries in Wrath, survival hunters and frost mages. Both will return a percentage of a player's base mana, not a percentage of the caster's damage. What this means is that the mana regeneration is predictable and controllable at all levels of the player's progression. It also severs the link between damage and utility that holds shadow priests back.
The surprise is that Blizzard hasn't carried the lessons from this into its design for shadow priests in Wrath. They have nerfed the shit out of vampiric touch and claim we are still returning similar amounts of mana.
If that concern is holding Blizzard back from making some meaningful improvements to shadow priest dps, there's a very simple solution: tie vampiric touch to base mana, not damage done.