I seem to be part of a definite minority here. But...what's so powerful about a feat that grants an extra domain? Now unfortunately, this can only be answered by opinions, so let me try something that worked *against* me a while back.
It's hard to argue against the fact that clerics in general are overpowered. Yet, though a party requires them, the majority of players hate playing them, for various reasons (walking band-aid, must buff up to kick butt, must adhere to deity's tenets, etc etc). Therefore, the two facts sort of cancel each other out. Looking at balance alone, clerics *scream* to be nerfed. Looking at how much they're needed in parties, and how few players actually want to play them, and you sort of have to decide to leave them alone.
So. Let me try that with this feat. Has *anyone*, *anywhere* actually had a PC cleric take the split version of the feat? (So he gains either the granted power or the spell access, but not both.) Has *anyone*, *anywhere*, actually had a PC cleric take the feat that grants both--at the epic levels?
The feat is far far too underpowered when split. While I'm not so sure about epic levels, never having played them before, in comparison to other epic feats out there, it seems underpowered as well as an epic feat, even gaining both abilities. Although perhaps not as extremely as the non-epic version.
The one concession I do give is that I do agree that it's slightly too good to be available as a 1st-level feat. I give my players access to both abilities as a regular feat, albeit accessable only at 5th level or higher. *shrug* I don't see it as overpowered at all. And no one's taken it yet.
Extra Domain [General]
You have access to more domains.
Source: Adapted from any one of hundreds.
Prerequisites: Access to a domain; Knowledge (Religion) 8 ranks.
Benefit: You acquire one additional domain chosen from your deity’s list of available domains, including the domain power and access to all domain spells.
Special: You can take this feat multiple times. Its effects stack.
Notes: Most split gaining the powers and spells into two Feats, or consider this feat an Epic-level feat. I do not. You do not gain any additional domain spells per day, so what you gain is an additional domain power and more spells to share the single domain spell slots.
Edit: Heh. "It's hard to argue against the fact that clerics in general are overpowered..."