Companions and crafting
Unlike most other MMOs, where there are maybe one or two 'pet' classes, all eight classes in SWTOR have their own companions, all of which are unique in appearance and storyline, help you fight, and grow to love or loathe you based on your actions.
Each class gets five companions total, plus an additional ship droid shared on each faction, a lot more than the developers originally intended. These might be humans, aliens, or droids, with lots of variety to spice things up.
Companions get their own skills, which they will use automatically, and they can also be fully customised in terms of gear and appearance, with their own tab on the character page.
At first people playing the same class look like they're being followed by a series of clones, but you quickly gain access to cosmetic options that help differentiate your companion from an otherwise identical one belonging to another player.
Instead of heading back to town to craft, you send your companions on crafting missions, either to get raw materials, make an item, or participate in unique missions that offer things like companion gifts.
There are fourteen total crew skills: four that gather resources, six that craft items, and four that work like timed mini-quests for your companions. Crafted items can be traded, sold, or even reverse engineered for some of the raw materials and a chance at a higher tier schematic. Some recipes create unique items like reusable medpacs that bind to the character, giving each crafter an interesting bonus that they cannot acquire through the Galactic Trade Network.
This use of companions even extends to harvesting resources while you quest. You can tell your companion to scavenge some droid parts while you run off towards the next objective.