The first hands-on previews of GTAV are coming out today, with various European gaming sites getting to play the game first. Below is a short summary of some interesting points that were mentioned in some of the reviews. Most other details concern some of the early missions, so with the interest of not wanting to spoil anything for anyone, we've left those out. If you don't mind reading about the early missions, then check out the links at the end of the post and read the whole articles.
 
Summary

• Missions are extremely fun and varied
• Characters will sometimes engage with pedestrians in totally unique ways to immerse players deeper into the world, for example, IGN saw Lamar randomly accost a drunken pedestrian
• While on the ground, you feel like a tiny part of the game, while seeing Los Santos from the air you get to see just how vast and expansive the city is.
• No graphics stuttering, and the infamous popup scenery that has plagued the GTA series since GTA III was only seen a handful of times - it has drastically improved since GTA IV.
• The idea of character switching is a fantastic weapon, but if you use it right in-game you'll see how it can be extremely effective.
• The diversity of the city, its surroundings, the characters and the whole world is really amazing.
• There is a "colossal range of vehicles"
• Police are a lot more intelligent and can spot you from a distance
• The weapon selection wheel will store all your weapons, even when out of ammo, and you will never lose them.

Quotes from IGN:

The distinction between GTA V and GTA IV’s vehicle handling is immediately clear; carving my way through the throngs of 18-wheelers the low-slung supercar feels more sure-footed and grippy than similar models in GTA IV. It’s more responsive, but without erring towards feeling too superficial or light.

The motorcycles have benefitted from the handling improvements too; they still feel quite manoeuvrable but, at the same time, they now come with more believable sense of mass.

I'm struck at just how much brighter GTA V is than GTA IV. The harsh, San Andreas sunshine lends vehicles a dazzling gleam, casts sharp shadows, and gives the whole environment a brilliant, suitably over-exposed summertime feel. Aesthetically, Los Santos stands in stark contrast to the somewhat washed-out bleakness of Liberty City.

 
Screenshots

 
 
Hands-on Previews

• IGN UK
• JeuxActu (French)
• GamesAktuell (German)