Which Schools of Magic should I use?
What I consider the most important spells are levitation (Thaumaturgy) and recall (Mysticism).
Mysticism + Thaumaturgy also help to cast spell reflection, which is extremely helpful at higher levels.
Otherwise, Restoration can be pretty useful for healing here and there, Alteration for if you ever plan on swimming (I avoid water at all costs, personally), and Destruction for damage.
Which factions beside the Mages' Guild?
A druid may be interested in making potions, but that requires joining a temple (not Kynareth or Julianos). Arkay allows it at rank 4, the earliest.
Are language skills worth it?
No.
Are pacify spells (is that the right name?) worth getting?
I don't think so.
Should I use weapons (like maces, daggers) instead of my fists?
As in many of the Elder Scrolls games, using hand-to-hand allows for rather fast leveling when used as a primary weapon. It can be useful, even better with critical strike. Also, you won't be breaking a weapon while forcing open doors in a dungeon. Unarmed attacks ignore monster material immunities. So, if you're fighting a vampire, for example, you won't need to pull out a silver+ weapon.
If it's only a fall-back skill, however, you may want a heavier-hitting weapon instead.