The actual gameplay is typical Rockstar, slightly clunky walking and driving, the latter suiting the late 1940's. Some damn good looking cars were made back in that era. Unlike GTA or RDR you can't just whip out your sidearm and blow away pedestrians - you are part of the LAPD. Even Nick Kang had a karma system to stop him doing this too liberally, Cole Phelps must get by running over the occasional street walker and get a few points deducted from his mission record.
^ Cole, the hero and the villain (ha)
I kind of liked how they melded the flashbacks showing Cole the by the books officer in the army being a total disaster (well, it's complicated) and being an absolute star in the police (with your help). Nice kind of redemption to it, though it all kind of goes to crap as you can imagine.
The actual interrogations and gathering of evidence is the absolute best part of the game. Any car chases can be painful - don't skip driving to destinations, you'll get rusty. Foot chases can be intense. It all adds up to putting a case together, getting the questions right using the right evidence or intuition* and hopefully putting the right criminal in the slammer.
Almost every case in every department is entertaining in some way or another. I actually think it runs out of steam during the late Homicide cases of all things and into Vice. Cole is best as an exceptional underdog rather than as the certified best. When he loses room to maneuver in the late game we get introduced to someone so frustratingly good and perfect he makes us miss Cole dearly.
^ Who's the best partner? Stefan is a great help at the start and fully deserves to be where Cole should stay, in Homicide, later on. I actually really love those first 3-4 cases in Traffic. Rusty 'helps' you out in Homicide for the whole 6 cases he is entertaining but wholly useless; thankfully they all drive and chase the same way and are all equally useless in a firefight. Roy Earl is the worst, fun to see how he moves about like the corrupt cop he is. I sat in on his little 'meeting' with the chief there. Bastard. Herschel Biggs is great and just the right guy to help Cole when he's down - though it takes him a while to get there.
I mainly posted this as I realised I had a heap of screenshots (well, 34) from my recent playthrough on Steam. I have played it a number of times on the 360, it was one of my absolute favourite games of recent times. I'll hopefully come back to this and expand on it a bit more as it deserves. Love, love, love.