Mage's Guild advancement stands out because it is not advanced by it's own quest line or any repeatable grind, and takes way longer than any other quest line, except maybe Cadwell's itself (though I think it's even longer!). It has been estimated elsewhere in these forums that it would take about ten zones, two complete factions, to finish. The books required are not repeatable. The only way to 'grind' is boring travel (though some books take a bit of fighting to get to) if that's all you're doing - and why would you? Mostly, you wait and do Cadwell's quests and grab the books as you go, making advancement in the guild drag on for weeks. It is inconsistent with every other form of advancement in the game, and really makes no sense. Before One Tamriel it made even less sense. It may be just a point of view thing - maybe if you didn't grind, Fighter's Guild would take just as long. But seeing all those books, especially when it's your third or fourth character and you know how long it will take... Disheartening doesn't really cover it.
To make Mage's Guild advancement a little more equal with all the other guilds, two ideas come to mind. One: increase the points you get from books in some fashion. Adding books in faction zones would work, but I'm guessing that won't jibe with either players or the developers. Increasing the points per current book could work, but better, I think, would be to give points for other books in the game. This could be some set fraction of a point for each non-mage's-guild book (repeatable/grindable???), or add rare books for a large chunk of advancement. Or possibly some combination of those. My second idea would be less developer work: Add the quest line experience to guild advancement. For the Psijic's, the main quest line advances you just so far, then the 'collection' phase finishes it off. For the Mage's Guild, just add a percentage of the quest line experience to guild advancement along with the books. Much simpler and easier to implement. For consistency you'd probably have to do the same for Fighter's Guild - not a bad thing, in my opinion. Grinding undead or daedra is a bit less monotonous than grinding books, but still a grind.
Please Note: Yes, I know I'm a consistency knob. While I do believe that such a change would enhance the game experience for everyone, I know the consistency bent is my view, and I don't expect everyone, or anyone in particular, to agree with me. I'm sure the developers that designed the Mage's Guild line had reasons (Who knows? They could have initially designed it to be completed in one faction, and someone else decided it was too many books for the database, and boom! Unintended consequences.) Whatever the case, it's a bit of a slog, and I'm hoping it can be shortened.