Fighters Guild: A main quest could be to go and clear a cave of a goblin infestion, but to take that quest the leader would require you to have a certain amount of skill. But a offquest may be to deliver a package or a letter from place A to B, or to clean the guild hall or cook meals for the fighters and such (some offquests could even need skills like if you could cook then you could take a cooking quest but else not).
Mages guild: A main quest could be to go to some ruin and decipher some runes (you would maybe need X amount of intelligence for that) or to add enchants to the guard captains armor. When a offside quest may be to go and collect plants for the alchemists or to prepare the guild house for rituals and such.
Thieves guild: A main quest could be to steal some artifact while a offquest may just be to deliver a note to a certain place at a certain time.
Dark Brotherhood: A faction like this one may be a bit tricky... they don′t mind you dying but they don′t want you to potentially screw up on a mission and to uncover their hideout, the Dark Brotherhood could only require loyalty to join and then hand out different assignments based on skills, but you should be able to rise up in ranks in it, be it that you are a figther type, a sneaky agile type or a magi.
And I can go on and on and on for different guilds but I will just stick to the 4 main guilds in Oblivion.