Story vs gameplay in GTA IV

Over the weekend I completed GTA IV. Or rather I completed the story mode; I'm only at 65% on the progress bar but I really can't be bothered to go round shooting all the pigeons and whatever else I need to do to get 100% completion. I've done a couple of races and a couple of assassination missions and a couple of everything else, so I think I've experienced all the basic experiences the game has to offer, even though I haven't had all the variations.

What can I learn from it as a game designer? I think the game's clearest failing is that the story missions and the rest of the gameplay didn't fit very well together.

Firstly, the story was quite linear, whereas the wide-open sandbox gameplay is fundamentally non-linear. Secondly, the tone of the story doesn't fit very well with the tone of the sandbox game. The tone of the story was serious and dramatic, whereas the sandbox game is, if not actually comedic, at least more light-hearted and fun-loving.

I like to roleplay in games; I like to feel like I am stepping into the shoes of the lead character, and playing their part in the story as it unfolds. The story cutscenes and dialogue in GTA IV provided me with a very nicely realized character to play, so during gameplay I was thinking "What would Niko Bellic do?" rather than "What do I feel like doing?". But I soon realized that roleplaying in this way meant voluntarily missing out on the fun of the sandbox game. To start off with, roleplaying as Niko Bellic, I would obey traffic regulations unless I had a pressing reason not to; I would avoid stealing cars (I used Roman's taxi extensively in the early part of the game); and I would certainly not casually kill people (the first time Niko kills someone in a cutscene, he comments that he had hoped to avoid that lifestyle in America). A key part of Niko's character is that he did not come to America to be a criminal, but dragged into that life as it's the only option available to him. That's a compelling character, but in a crime-spree themed sandbox game roleplaying that character means deliberately trying to avoid the fun bits of the game.

So eventually I found my behaviour changed and I started thinking of GTA IV as two separate games. In the sandbox game, I stopped roleplaying entirely and just drove around doing fun things. Those fun things included missions, which I thought of as a separate game. The storyline applied only to that game-within-a-game, and not to the game as a whole. Sandbox-Niko had no real personality or relationships, and did things like steal multiple cars and drive them at full speed into a river in order to complete a stunt jump. Story-Niko committed criminal acts grudginly for money, and had relationships with other characters involving love and betrayal. In between missions, Story-Niko presumably obeyed traffic regulations and did grocery shopping and ate three meals a day even when he wasn't wounded.

Possibly the game as a whole doesn't suffer too much for having to be understood in this way. I still did feel moved by the story's dramatic moments (including the ending), and where it required me to make a choice I really did think 'what would my version of Niko do'. But I couldn't help feeling that I really had two different games here, and maybe they would be better played as such.