If you already have a high level of skill (borrow loads of money and train yourself) and if your personality and past deeds ensure you have a high reputation before you join the Guild, nobody will ever ask you to kill a rat. They will straight away ask you to handle a lot more complicated tasks. And if you do handle those complicated tasks like banishing daedra, you get higher reputation point rewards.
For example, if you have already made a strong reputation as a Brother of the Temple of Julianos, and if you already have developed a high level of skill in Alteration, Mysticism, and Thaumaturgy as a Brother of Julianos, then when you join the Mages Guild, you will straight away begin as a medium-to-high ranking member, and straight away get the really challenging tasks. Your reputation from other guilds carries over. Nobody will ask the Patriarch of a temple to wash dishes and clean out rats if he joins any guild. And nobody will ask the Archmage of the Mages Guild to run chores for a temple.
I have checked this myself several times. I got promoted to Archmage of the Mages Guild without doing Mages Guild quests. Why? Because I regularly did favours for friends of the Mages Guild - namely the Daedric Princes. Doing daedric quests over and over made my character highly reputed across several guilds.