This is an issue that has been brought up, worked over, rehashed, torn apart, abandoned, and reawakened by practically every WoW Blog out there. So who am I to mess with tradition? I have to say, I like the ideas I've seen so far. Everyone seems to be in agreement that your guild's various triumphs should be made visible by visiting the hall. Whether having Ragnaros' mighty hammer on the wall or having Deathwing's Jaw on the coffee table, you should be able to get a visualization of everything you've done. Perhaps even have the various items come with a cogwheel function where you can read who was involved in the acquisition of the items (as in, which members were in the raid when you first killed some boss or other).
Sadly, however, the guild hall ideas seem to focus on raiding guilds. I don't recall seeing too many guild hall ideas that consider PvP guilds or RP guilds or even just social levelling guilds.
So I've got some ideas there.
PvP guilds need to have some kind of display board where they can see various arena rankings of their members, or their RBG ratings, or even just body counts. Also, said guilds should be able to display trophies of their completion of various PvP objectives. This extends from the various battleground objectives (like killing the faction leaders in Alterac Valley or collecting the flags from Warsong Gulch) to non-BG objectives (like killing Garrosh and Varian). Trophies could be simple. Garrosh's shoulder piece (the big horn things), or Varian's sword. Or they could be heads on sticks.
And they need to include PvP objectives from all over, including places that seem to have been forgotten, like Halaa or The Bone Wastes in Terrokar Forest.
RP Guilds need to be able to collect and display items that go well with either the guild's overall story, or just relics from individual stories (like a Brewfest Pony Keg to remind everyone of that time you got drunk and tried to tank Naxxramas without pants or a shield).
And social/levelling guilds need to be able to display their various types of progress. Whether it's a small box of flowers to signify how they're doing on their herbalism or an armor display case that shows what their average character level is, they need to be able to show where they stand.
As far as individual player housing goes, I think that players should be able to display anything with a model attached to it. Whether it's a weapon on the wall or armor on a rack, it should be displayable. Also, they should be able to put various toys (like the stuff I talk about here from time to time) on shelves to display them. Think the Wonderworks in Dalaran. If a player is into PvP, let their house show their various achievements in the field. If they're a raider, let them display some smaller trophies from the raids. If they're an achievement junkie, Light help them, they're probably going to need a bigger house.
I, like so many other bloggers, know full well that Blizzard will probably never implement this stuff. But it's fun to imagine it, even if only for a few minutes.