Here's how to max out your daily guild experience: Run heroic Shadowfang Keep with a full guild group. Why Shadowfang in particular? When Baron Silverlane dies, all his living adds despawn. Then you are awarded guild experience for him and for each add. I can't claim to understand what it is in the code for awarding guild experience that would cause this. If you kill the adds before he dies they do not award guild experience, not all bosses with adds give guild experience for adds that are alive when they die.
Anyway, if you manage to not kill any of his adds at all, which is not that easy to do since the wolves that come with Nandos have very low health then you get experience for eight bosses all at once. Heroic bosses are worth 37.8k. With the 25% bonus for having five people that's 47.3k. Eight bosses makes it 378k for each person in the group for a total of 1.89M guild xp for that one kill. Since you killed Ashbury on the way for another 236k, you end up earning 2.13M guild xp in probably 15 minutes. Do this with three groups and the cap has been reached.
Similar guild experience errors exist in other places. Omnitron rewards four times the guild experience that most raid bosses do. A single Omnitron kill in a 10-player raid gives over half the daily experience cap, though obviously you can do that only once a week.
I bet Halfus could be killed with his adds for bonus xp, though killing halfus while his adds are still alive would be a bit tricky.
But it's the SFK bug that bothers me the most. Here is the question: If my guildmates and I want to run a heroic to get some guild xp - since we don't really run heroics for any other reason at this point - should we queue up for SFK or not? Obviously the extra xp is a bug, and so by using it we are exploiting a bug. But what is our responsibility to avoid exploiting this bug? Should we queue up for the next best heroic for guild xp (probably Halls of Origination) instead? Should we queue up for random and secretly hope for SFK? If we get SFK should we kill Baron Silverlane's adds before we kill him to avoid earning the xp we know we aren't supposed to get?
But maybe underneath all of this is the real problem that we don't particularly like earning guild xp. The methods of earning guild xp are remarkably narrow, and running heroics is really the only way to get to the xp cap for the day for a smaller guilds. The fact that guild xp is capped daily means that raiding will never be a good source of guild xp, no matter how much the bosses are worth. Since many guilds are formed for the purpose of raiding this seems somewhat counterintuitive.
I know that the developers said that they didn't want to give guild experience for a lot of things like making flasks for or providing enchants to guild mates because they felt these things would be gamed. The developers have to worry about systems being gamed because the large number of well known information websites about WoW means that the optimal path to guild xp will spread through the community quickly. If making flasks was how you max your daily xp, then someone would be expected to log in each day and make the flasks.
The problem is that guild xp is primarily rewarded for something that guilds don't necessarily do. If having someone log in to make flasks to max experience seems bad, I don't see how having people queue up for SFK seems that much better. At least the flasks are actually helping the guild.
In an effort to create a system we couldn't "game", the developers have created a system that is far too restrictive and doesn't represent the purpose or values of a large variety of guilds. The broad base of players may have been better off with a system that could be cheated, if it also let them earn guild experience by doing the things that their guild does.
As it is this complaint may not end of mattering, since most guilds will hit maximum level in about a year and guild experience will become a thing of the past.