I always saw it as Joyce being just as much a part of Buffy's foundations (get it?) as her Slayer lineage. Her maternal human line is as important as that of the mystickal.
Dreaming of a dirty wall (Buffy, "It looks dirty.") usually means that your body needs cleansing because there is a toxin. Since it is Joyce surrounded in the dirty wall "body" it can mean that it was a foreshadowing of her illness.
Joyce told her to leave her and find her friends because in the end she will not like that she is the Slayer but she will accept it. When she gets sick she is concerned that Buffy is putting her life (friends, school, boyfriend) on hold to take care of her. In Restless, Buffy was not choosing hanging out over helping her mother. She had to find her friends who were in trouble and needed her. She was choosing the perils of the paranormal over the hardships of everyday humanity.
A wall does represent a seperation. Buffy has always felt that she was letting her mother down and had to shut her out of so much of her life because she knew she couldn't handle it and didn't condone it. I think it says something that Joyce is trapped in the walls of the university. Buffy will have to give up college and all of the dreams her mother had for her. She moves past the belief that if she tries hard enough she can have that life while being the Slayer.
Some say that dreaming of a mouse symbolizes a serious health threat, such as cancer. So once again we come back to Buffy being warned of her mother's impending illness and death. Joyce said that the mouse "cancer" was tickling her foot. In Bargaining #1 the Buffybot gets injured in the head and says "I think my foot is broken."
I don't think Buffy took her mother for granted. She tried very hard to live up to the "normal girl" expectations of her mother. She had to hear that she was disappointed in her, that she made bad choices and showed poor judgement, that she was sick in the head and probably capable of murder. Joyce held a lot of resentment towards Buffy for being the Slayer and not following the path she dreamed for her (marriage, kids, a place in normal society).
She didn't see her that much in S4 but let's not forget that she wrote her every week, came home her first week of college to find her bedroom filled with shipping crates, came over and had a good talk during Halloween, would've celebrated Thanksgiving with her had Joyce not decided to visit her sister out of state, etc. They were often two stressed out busy ships in the night when they lived together so living apart, of course they won't see as much of each other. I didn't exactly see Joyce coming down to the university to spend time with Buffy, it goes both ways.
