Yes I myself made the mistake of keeping the main story front and center on my first playthrough and once I finished it the central opinion I formulated was "phenomenal game" but "too short".
On my second playthrough, however, there was so much content and quests I missed that took me by surprise, especially the companion quests which net you loads of xp on completion, that I genuinely felt stupid (I usually play as solo chars, managing companion abilities and personal abilities simultaneously annoys me).
Currently am playing on the hardest difficulty as a solo character and if you actually take the time to raze through the worlds and quests, there's still a whole bunch of side content that keeps you going, even w/o companions!