One almost constant criticisms of the Marvel Cinematic Universe is the quality of its villains (Loki aside). It all comes down to a multitude of sins: Antagonists like Ronan and Malekith are not given enough time to develop properly as characters, and therefore come across as bland and forgettable, while there are too many that are simply a mirror image of the hero (Iron Monger, Yellow Jacket, and by the look of the Black Panther trailer, Erik Killmonger).
Thanos, however, has been built since Avengers Assemble as a step above the rest, labelled as the 'big bad' of the franchise. With this title comes a pressure to be better than Marvel's other villains, upping the ante and the threat to our heroes.
There will come a time (soon) when The Mad Titan is defeated by The Avengers and co. and with Marvel showing no signs of slowing the MCU down any time soon, this will leave a void for another 'big bad' to fill.
In the build up to Infinity War you would be forgiven for thinking that Thanos is the worst or the worst in the MCU. Yet the universe is full of evil beings that can threaten the world just as much, if not more than The Mad Titan.