His father "defected" in the sixties to help bring back technology to the Soviets. when Tony's dad realised he wanted to use the power technology for "bad", he basically had him deported, which didnt please the soviets as he came home without any advanced tech so they shipped him off to Siberia where he lived out his life in an alcoholic stupor. His son blamed the Starks for his fathers plight, felt his father was betrayed (since he helped Howard stark build the original arc generator, how his fathers contribution had been removed from history), and his hatred ended up being focused on Tony Stark. He wanted to make him suffer rather than out and out kill him, because he wanted to discredit him in the eyes of the public.
It's all there in the film, and not exactly hidden beneath subtext.
We see a Tony Stark who is basically dieing, slowly loose the plot, that is why he does all the irresponsible stunts and crazy acting out. Around this central story we see his frienRAB briefily loose faith in him, we see Whiplash's reasons for wanting to hurt Stark, we see his business rival want to discredit him, we get a better look at Shield with Fury, we see tony figure out the message his father left him and develop a new element, whichi n turn lets him repower himself without the fear of dieing, which brings the old Tony back, we then see an all out action scene at the end when many, many drones attack and chase Stark before he and rhodes take them down in a fight, which is before they face off against Whiplash one more time.
Is the film filled with all out action moments dashing from one to the next? No. Does "not much happens in it" describe the film remotely? again no.