I'm pretty sure it's BtB that clerics get a whole range of prayers (spells) at their disposal. They may only be able to use one first level, but they have the whole list (in theory) of first level spells to choose from. So, that makes sense to me. I'd also like some cantrip like abilities (called orisons in Dragon Magazine) that would be available at will.
Clerics get to turn undead and they have a lot of weapons and armor available. If they don't advance in fighting ability, they need to advance in abilities like turning.
I'm thinking of giving clerics a variation on bless - to increase as the cleric increases in level.
Clerics will also get to make and use scrolls and relics.
I guess the one area that I would houserule on clerics is that they would have a chance to succeed in a prayer/rite/spell that is higher level than they can currently pray/accomplish/cast. With diminished chances for success, of course. So - like one level higher might be at only 50% chance to succeed - 2 levels higher 25% - keep halving it? 3 levels 12%, 4 levels 6%, 5 levels 3% and 6 levels 1%... I'm not sure if the 50% isn't still too generous. I'll have to think more on that.
In Daen Ral I'd like PCs to belong to the Monolithic, Monotheistic church of the Way of the Master, giving them some specific abilities geared toward that faith. Druids and clerics of other faiths exist, though their powers are varied from the standard "cleric" which is of the Church of the Way of the Master... Druids, for example, draw on nature and can accomplish some powerful magic as rituals (especially when combined with other druids), and clerics of the Old Gods or the False Gods will have powers specific to their deities (the Old Gods being elemental beings, the False Gods being demonic beings - the Firstfallen of the Host of Heaven).
So, clerics as a class are generally analogous to Medieval Catholic Priests...sort of...
Oddly, considering the fact that I played a number of clerics in the past, I don't have much to say about specializing them...