From the point of Class into class in final fantasy history, the most iconic thing I can think of regarding a thief is that they almost always are the prerequisite to become a Ninja. In the Tactics games you needed thief to become a Ninja, and in 1 and 3 the Ninja always seems to be compared to the thief since they share similar stat priorities. The ninja is usually refereed to as the advancement of the Thief and the thief is usually an under powered Ninja.

Their most iconic battle prowess, while usually its their signature abilities...which could pose little to no advantage on FF14, their most redeeming battle capacity is in their stats. Both are the two fastest characters and both have high evasion rates.

I don't really see Thief being a Job. Though there seems to be a hive mind mentality that the Classes are completely useless once you get a Job, a Job itself is nothing without the original base class it expanded on. I can see Thief becoming the base class and still holding its normal status without the need to short change its possible job the Ninja. Ninja won't be much without the Thief Class there as its base.