Finished GTA IV (by which I mean I beat the last mission, still have to get 100% completion).
What I liked: The car controls are just fantastic. Being able to handbrake drift curve is just tons of fun, and it makes doing the crazy stunts and crashes easier (my favorite car is the Sultan RS, total rally car fun). The copter controls are a bit more difficult than they were in San Andreas, but it's probably more realistic. I understand the lack of being able to fly planes, but I wish they kept the jet pack around.
Niko is the most well developed protagonist in the series thus far. The guy in GTA III literally had no personality; Tommy in Vice City was just a rather stereotypical mafioso; CJ in San Andreas had a more interesting angle, but it was still rather straightforward gangsta. Niko's got this serious, tortured soul going on. He's not killing out of bloodlust or machismo, it's the byproduct of the war, the only way he can live life now. His scene with Darko was downright powerful.
The radio was successful, although not without its problems. Their seemed to be too much repetition of styles across the channels: three talk radio, two rock, two reggae, three dance/r&b, two hip-hop. And a lot of them were set-up in the same manner as San Andreas (classic rock vs. alt rock, classic hip-hop versus modern). The new age channel was a complete wash. Still though, they had some really good stuff in there, Queen, Justice, Nitzer Ebb, Sisters of Mercy, Killing Joke. And the Russian channel was great.
What I didn't like: I get that the scale needed to be scaled down with the serious graphics update from San Andreas, but I still felt like it wasn't as epic as its predecessor. Like I was playing a "smaller" game, in that sense.
Okay, Liberty City is obviously modeled on New York City, it's supposed to feel like you're exploring the Big Apple. And it did a great job with that. However, (no offense to Bacon) it seemed that, with that in mind, it dedicated way too much space to the Hoboken/Newark Jersey area. I would have rather they didn't include it at all, and expanded out the Brooklyn/Queens island and Bronx island (especially the Bronx one, it was very undeveloped).
It's a bit buggy, or at least the PC version is. The final mission (if Niko doesn't take the deal) is impossible to finish unless you do some seriously odd things (I had to assign one core to the game, something I wasn't even aware you could do on a PC). And if you try to start a vigilante mission where the baddies spawn, it freezes up the game.
"How is it that hardly any major religion has looked at science and concluded, 'This is better than we thought! The Universe is much bigger than our prophets said, grander, more subtle, more elegant. God must be even greater than we dreamed'? Instead they say, 'No, no, no! My god is a little god, and I want him to stay that way.'" - Carl Sagan
"To doubt everything or to believe everything are two equally convenient solutions; both dispense with the necessity of reflection." - Henri Poincaré