This is pretty much like the one in the GTA V forum. Although the game's nearly 6 years old so I guess most things about it are already known, but then again I always discover something new. Maybe there could be a few things not everyone knows, but either way it's still cool to discuss.
Anyway I'll start.
-It's possible to use the internet Cafe before "Logging On" thus contradicting Niko's claim during the mission that he hasn't logged on "here" yet.
-Liberty City's very cold at this time of the year. You can see Niko's breath in the air everytime time he breathes.
-The opening credits bear a striking resemblance to RDR's as Niko and John both arrive at their destination via a boat and cars are lifted off. It's likely R* San Diego used GTA IV as inspiration. Or it's just a timely coincidence.
-Sometimes after an intense situation such as a shootout or a fist fight Niko will usually display an angry facial expression which goes back to his default expression a short time after.
-Various drug dealers are around Liberty City. Mostly in alleyways and sometimes in Outlook Park. They offer their product to peds. Some peds will buy while others will refuse angrily usually telling the dealer to "f*ck off".
-In GTA IV if you visit the site of the Hercules nightclub (TBOGT) in the vanilla game it's just a random texture.
-Police officers can buy newspapers from newstands.
-Girlfirends are very noticing of what Niko wears/drives. Not only will they comment on his clothing/car, but if you visit them again with something different they usually say something like "I miss those glasses" or "this car is a big improvement over the old one".