Pro Carefully-executed changes to existing maps
The updates and improvements made to existing maps have been pretty clever and useful.
For example, cracked glass is more opaque, making it harder to go on a sniping rampage. Or the stairway which was added to the bottom of de_dust made the route more viable for the Terrorists, while keeping the original purpose of that area serving as a bottleneck.
Pro Driven by tactics and cooperation
Each round of CS feels more like a game driven more by careful tactics than a frivolous fast-paced and lethal paintball arena of which too many modern First-Person Shooters are guilty of.
Positioning, timing and thinking are key ingredients of a successful game and cooperation is crucial if you want to win.
Pro Will be familiar to CS 1.6 and CS: Source players
Counter-Strike: Global Offensive builds on the strengths of its predecessors, even though those predecessors have been constantly improving for over a decade now.
Nonetheless, Counter-Strike veterans will feel right at home in Global Offensive, albeit with some minor differences. The MP5 is now the MP7, The TMP has left its place to the MP9 and you can't attach a suppressor to the M4.
Con Less time spent playing a game
This is a feature that makes kills and hits (no health regeneration) more impactful, but also a lot less time is spent playing the game, especially if you die early to a sniper peek. Newbies will be playing a lot less of the game than more experienced players.
Con Hard to see if you are hitting someone
One of the things that were changed in CS:GO is firing feedback.
When someone is shot in GO, they don't wince, there's only a small amount of blood and audio that conveys the information that you are actually hitting someone. But omitting wince animation from character models makes it harder to see if you are actually hitting another player, especially at distance.
Con Can have a bad community
As it is a large game, it should come as no surprise that the community just won't be up to par. You may be kicked for strange reasons (like bottom scoring or the all too common random hacking accusation) and many could give you a 7 day ban. It can be very frustrating as missions in the mission system require you to play certain things while crossing your fingers.
Con New maps and modes feel pretty mediocre
Even though old maps have been revisited and improved by a wide margin, new maps and modes released with CS:GO are pretty mediocre.
Half of the maps available are locked under Arms Race (which is a re-imagining of the original community-created GunGame) and Demolition (GunGame without insta-respawn and bomb defusal) modes.
These game modes feel pretty safe and unimaginative, especially for veterans who have played their predecessor.
Some of the new maps are pretty compact and designed to act as instant-action meat-grinders akin to modern FPS (like Call of Duty) instead of requiring any tactics to win.