I find that Warlocks can do a bit of what other classes can do, making them versatile, but they are not the only versatile class. Bards are even more versatile.
However, Warlocks are incredibly distinct if you use the lore and story of the class. That patron relationship is absolute gold for role playing. They are subject to a pact with obligations and benefits. Each pact can be very different. This is a different relationship than a cleric or paladin worshipping a God, or a druid revering nature ... this is a business arrangement, pure and simple. Well, not so pure, usually, and not so simple, either.
As a DM, when a player says they want to play a Warlock, I salivate. It is gold. My next campaign is starting in two weeks and I am looking forward to the player that decided to play a Fiend Pact Warlock. Old School. Classic. And I have such a good spin on it for him - based upon the backstory he wrote.