Having played through the game myself, I have little to say about the side quests, which is unfortunate, since they're a nice bulk of the game. However, I became a bit drained by SMTIV as I played through it- the atmosphere and lack of character in the close party of characters made the surprisingly awesome story a bit difficult to trek through. I also took a bit of a break in the middle of the game which was a harsh blow to the momentum.
The opening moments of SMTIV are captivating, in my opinion. The fight through Mikado and revelation of the Overworld were so unlike anything I've seen from an RPG (apologies, I must admit I've never played 1-3) and the bleak tone of the world was very interesting to me. I also thought the story twists and turns were quite good, though the humor mostly fell flat in a majority of the localization. The main group of characters you interact with aren't very fascinating, they exist to serve as extremes to help you make your ultimate decision. It's the extended cast, really that of the entire Overworld, who are really full of character, and that's where the majority of the story takes place, so that's good.
The gameplay was wonderful, though I wish the game didn't assume players are familiar with all the possible demon fusions, because you can miss out on some awesome ones if you don't understand why certain demons are useful despite their pointlessness in battle. The press turn system is delightfully exploitable and punishing, and though it makes the game a bit cheap at times (I struggle to use the term difficulty because most of the hard parts are more trial and error, even with an excellent Flynn build), it also ensures that you are always on your toes, which is a good feeling.
Like TOP said, the soundtrack in this game is out of this world and really adds to the atmosphere, and I would argue the game has some of the best presentation you can find on the system, with glorious art and environmental design. The harshest flaw I can give the game is that, as soon as it opens up, it also becomes extremely ambiguous, and some of it's directions make sense for someone who knows what the Overworld is based off of, but for others might be a bit brutal. Even so, I enjoyed the game enough to lump praise on it and be really excited for this next installment.
I also got a Law ending and I'm looking to do a Neutral ending in my New Game+, which I really believe is exactly how Atlus wanted the game to be played.