I can't stress more that this would be self contained. This is not meant to be a personal guild ship. The guild ship needs to have it's place. It's not an imbalance in the game, just that we don't want to remove the appeal of being in a guild. I never joined a guild before as there was no point, no payoff in the end. Now with the buffs, discount purchasing, and transport to different locations, it makes being in a guild worth it. You could still have buffs of a sort, but only those that work within your own realm. By having the castle/keep/stronghold only on each player's computer rather than the servers, you don't have to worry about over population or clutter that is abandoned by players. If you never go back to it, it would be just taking up space on your computer not their servers.
Wherever it originated from, it's concept is not without promise. The whole idea that you had something that is yours to do with as you want is appealing to many. If you don't want it, you don't have to invest the time. Those who do can spend time/platinum/money to their heart's content.
As for access to account rather than each character, I might agree if this was something that later could be made per character. I want my orc fighter to have his orcish stronghold full of barbaric orkyness, while my elven mage would have a nice crystal palace full of plants, trees, and meadows. It could almost work how the bind points on the guild ships work. You have limited amount of space that you could build and you have to decide weather you want a workshop here to create goods to sell for trade bars, or an armory that would improve the offensive/defensive capabilities of your troops. Have the amount of bind points available dependent upon your leadership and you could buy more bind point types with trade bars. And much like with guild ships you could buy more than would normally be available to you with TPs.
And many of the other posters have stated it would also balance out the OUTRAGEOUS prices that people think that they can charge on the auction house. Seriously, +6 items are not worth a million platinum people! I made a hat with +6 during the anniversary festival and it seriously didn't take that much time. I was able to do it for 6 toons and 2 of those have 2 +6 stat hats. Come on. A million platinum? It would have a nice economical effect for the wild pricing that happens. At best I'm willing to pay twice to three times the cost of an item, IF it is a perfect fit or perfect item mix, like a +5 Ghost Touch Metalline Heavy Mace of Disruption. I may pay three times the base for that if I had planned to do the Necropolis series. In any event I would pay twice that for it at least.
The more we talk about it the more potential I think it has.
Sorry about the non-topic rant, but I had to get it off my chest.