I want to distinguish between the different organised religions. At the moment, I don't know what they stand for, so I don't see the point in joining them, aside from getting potions. So I'll just have to make it up. I'll try to give a few examples and you can do the other ones, elaborate on my own or replace them with your own better ideas.
General
Bretons aren't a particularly pious lot compared to the likes of dunmer and they generally consider regularly worshipping the gods to be the temples' responsability. However, the temples do play important roles in society and have influenced their own regions' values.
Temple of Kynareth
This one of the few groups which pays homage to multiple deities. Naturally, Kynareth is at the top, but priests usually pray to her servants, the lesser air spirits, when they need something. They believe that the air itself is alive and compsed of a hierarchy of these spirits with a base of 'tiny' non-sentient spirirts at the bottom. The needs and desires of those who seek the priests of Kynareth's services are quite worldly, including good weather for the crops and favourable winds for sailing. Burnt, boiled and powdered offerings are given to the air, many of which are fragrant. The temple uses of form of theurgy based on this rather than whatever of the mages' guild uses. For example, curing a disease is a type exorcism. The temple believes that diseases are caused by 'unclean' air spirits (mad, tainted by mortals, minons of the devil Peryite?). The patient must fully exhale these spirits and the healer assists in this, removing and dealing with the spirits and preventing the patient from suffocating in the process. While other healers dispute the theory behind this beliving that diseases may be spread by non-airbourne vectors, they can't deny that it works extremely well.
The temple is favoured by farmers (Daggerfall consistently has good harvests) and sailors. They are often at odds with witches who also interact with air spirits as well as Peryite sometimes. These are the usual targets of the Kynaran Order who trained in lightly armed combat, acrobatics and spirit theurgy, including destructive weather magic and flight. Nobles tend to like them because they almost never interfere in politics.
House of Dibella
Another worldy temple, these people praise Dibella for making mundus beautiful, enjoyable and worth living in. They do this by creating and preserving beautiful things which appeal to all senses.
The temple is run by critics and patrons of art rather than artists, who make a revered part of the laity. They also sponsor craftsmen's guild, but how much they are sponsored depends on the aesthetic standards of their latest creations. This has lead to the guild depending on the House of Dibella and a love/hate relationship between them.
A House of Dibella does indeed resemble an large, opulent house complete with a bedrooms, lounges, a garden and a kitchen and dining room. There is also a theatre and exibition rooms, but the artwork itself must be rented out to nobles for safekeeping who must agree to put it on display for others to appreciate.
Illusion is uniquely used in several artforms by some members. Originally, it was used for pyrotechincs and acoustic modification in plays and operas, but nowadays, it used in abstract moving light displays and musical instruments which use impossible timbres by mundane means.
The House of Dibella tends to charge dearly for its and is less appreciated by commoners than other temples. While dress codes are relaxed for them (commoners usually have a 'Sunday best' which is somewhere between what one would where to the ballet and a nightclub), they must use a 'tradesman's entrance'. Some higher members would like to remedy this by offering more affordable services including public plays and communal bedrooms.
The Order of the Lily consists of guards at the houses themselves (the doormen are far more polite than many in real life) and knights errant who go on romatic quests and occasionally throw their lives away in duels for the honour or love of a maiden.

I'm going to stop now because I've have other things to do like sleeping. I have few ideas for others including the Resolution of Z'en, but I'm having particular trouble with the Akatosh Chantry. He's the god of time. He's a dragon. http://www.uesp.net/wiki/Skyrim:The_Alduin/Akatosh_Dichotomy in TES:V has confirmed that he is indeed the alessian Akatosh, rather than Auriel, which I find a bit odd, because he's very much a cyrodillic god, while Kynareth and Dibella for instance, are from the nordic pantheon. And they sing a lot. What for exactly? And this may not matter, but it's only popular in Wayrest, Balfiera (run by high elves) and Bergama in the Alik'r. So please, contribute your ideas and improve on or one up mine.

And what about the necromancers? What are they up to aside from raising corpses, abducting liches and being sinister and all-powerful? What do they ultimately want? A better alternative to the dreamsleeve? Something bigger than that? Why do they consider death a reward rather than a punishment?