I think that Blizzard needs to implement certain tools to help the casual guilds overcome barriers that prevent them from raiding. A few tools would go a long way to making the game more accessible to all players. All these tools can be done out of game, but if they were in-game it would be a lot easier and intuitive to accomplish. The fact that a tool is out-of-game is itself another barrier.
Also, these tools don't necessarily have to be very complex. The bare minimum should be good enough.
1. Allow guilds to form alliances
The biggest barrier to endgame raids is the lack of numbers and class balance. The easiest solution is to form guild alliances, where several small guilds team up to tackle the raid instances. An alliance would be created at the guild level, with officers or guildmasters being able to commit their entire guild to the alliance.
The option to form or join an alliance would only exist if there is a level 60 in the guild. Otherwise, alliances would be overused in the lower game, and you'd probably start getting 10 guilds of 5 people forming an alliance that would have normally been a guild of 50. Alliances are aimed at the endgame, and should probably only be available at endgame.
Guilds in the alliance would get a common channel, and share a common schedule (see suggestion 2).
2. Have a schedule where officers can post raids
Raids need to be scheduled. If there is one place in-game where the schedule can be seen, it will be a lot more effective than getting people to visit a website. A guild needs a common schedule, and a guild alliance needs a common schedule. It doesn't need to be very complex. Date, time, and event is good enough. The ability to do sign-ups would be amazing, but is not strictly necessary.
3. Have a base loot system with memory
Almost every raid guild uses a memory-based system to distribute epic loot (DKP, Zero-Sum DKP, Suicide Kings, etc.). I think that if Blizzard included a default system, that would be good enough for most guilds' purposes. If a guild felt that Blizzard's system wasn't good enough, they can always make their own system, as they do now.
I would suggest Spend-All DKP as the default system. Every time an epic drops, each player gets a point. The loot box that pops up has two buttons: Spend and Pass. If you hit Spend, and you have the most points, you get the item and lose all your points.
It's a simple and relatively fair system. Blizzard only needs to keep track of one additional piece of information: the amount of epic points that a character has. As well, the system would work across multiple guilds or guild alliances, as your point total is now inherent in your character, rather than tallied by the guild.
4. Have a guild bank
A guild bank, accessible by multiple people and with contents visible to the guild, would help guilds pool resources in order to accomplish goals. This would allow a casual guild to better work together. Right now, guild banks tend to be extra characters created by the officers. Such banks lack transparency which--in addition to trust issues--means that a lot of members are not aware of the contents of the bank, and are unable to take full advantage of it.
I think that these 4 tools would go a long way to enabling casual guilds to do raid content. Each tool lowers the barriers to entry for a casual guild, and makes it more likely that more people will experience the raid content that Blizzard puts so much time into.