The fact the original Counter-Strike remains one of the most played games on Valve's Steam and, maybe aside World of Warcraft, the game that gathers the most players each day gives us a little to no insight on how this game works. Major online games like Team Fortress 2 and World of Warcraft are updated regularly, and it's easy to say that if these games received the treatment Counter-Strike has received these recent years -- no treatment at all -- they wouldn't be this popular.
So what makes Counter-Strike so popular and sought after? Its gameplay, of course. It might have been in its release but in today's standards the original is not a pretty princess in any way you look at it, but the solid, responsive gameplay remains intact. Shiny, snappy pretty games might sustain a decent amount of popularity for a while, but what makes people transform a month playing a game then moving to another into a daily basis experience goes much beyond that.
Wise men once said that making something simple complex is a worthy feat, but making the complex simple is genius. Counter-Strike might seem simple enough. You earn money from winning or losing (more if you win), buy the weapon you can at the moment, go around killing the enemy team while trying to achieve the objective. If you get hit in the head you're dead, if you take a few shots anywhere else, you're dead, grenades deal enormous damage, engaging more then one enemy means you're probably dead too.
See? Counter-Strike doesn't go easy on the player. If you die you have to wait until the round gets a closure so you can restart. If you find that boring this game isn't for you, if you find it strange that when games these days don't offer regenerative health and longer periods of action, this game isn't for you. If you often get sad when you can't go Rambo and dominate the entire team just so you can gloat in the chat thinking someone cares, this game isn't for you.
This game is a team based game that when not taken seriously doesn't just mean your team won't win, it also means you're screwed. One shot kills won't be too far between, for the best or for the worst. This is what makes people come back for Counter-Strike, it's a game that challenges the player and rewards high curves of skill. If you're picking this game for the first and haven't had much contact with first person shooters in the past expect to be dominated, kicked in the nuts, spat in the face and get continuously punched on the stomach. If you endure and learn the tiny bits there is to know slowly, enormous riches await.
This remake goes beyond what Source brought. Source took advantage of the engine to update the graphics and a few hiccups from the original. It took a while but it eventually fell to general public's grace. That's the kind of challenge a remake for this kind of game endures. Generally people looking for a more casual experience will disappear after a while, but hardcore players might choose not even play it since they've dedicated so much time on previous versions they might stick to it.
No, Counter-Strike: Global Offensive is not as popular and played as both of its predecessors, and no it doesn't mean it's a flop. Source went through the same process and now it's a well established game. The community will grow into it as the time goes by. The graphics are not just prettier, the overall atmosphere was remade, in each of the maps, almost all of them coming back from the original once more.
Valve has a difficult task at hand, you have to please the more modern PC users that are not equipped with 256 MB FX 5200 graphics cards anymore, the mid-high end has changed a lot since the original, and has changed a lot from Source as well. At the same time people are better equipped and more willing to have a deeper experience graphically, there are veteran players (also casual players that played the earlier versions) who won't accept some trends in today's gaming market. It simply won't fit Counter-Strike.
So you have to make something new, while it feels old. It's not as easy as it seems. Looking back at it I think Valve has done a terrific job. They maintained the general Counter-Strike gameplay mechanics, separated what before could be done in a secondary manner: Casual and Competitive. On Casual things are made simpler, more centered on fun. You don't bump into other players, you can't obstruct and get obstructed by other players, there's no friendly fire, there are less rounds to be played, less money earned from killing an winning and so on.
On Competitive the rules for official matches are taken in consideration with games with 30 rounds (initially it was 10 rounds for casual, now it's been updated and is set to 15 rounds), team collision turned on, full money earned, friendly fire turned on. In a world where not everyone is a Counter-Strike professional this mode can be pretty annoying. Your team can be good but that one person who has just started played could easily jeopardize things up with team killing, not a troll, just a new guy. Comprehensible but frustrating nonetheless. So the existence of these two main game modes are important.
During Counter-Strike's existence several types of game modes were created unofficially by players, this occurred with Zombie modes, GunGames, Surf maps and such. One of these, the GunGame was reviewed and incorporated into this new version as an official game mode. You start "poor" handling pistols and grow up as you go through shotguns, sub-machine guns and rifles. You must get a kill with the weapon so you level up, earn the right to use a better weapon and become one step closer on winning the game. The last weapon used is always the knife, when you finally manage to get a kill with it you're the winner.
Another similar mode appears in the game except it's played in regular rounds and you upgrade based on how well you and your team are doing. Newer additions like these ones are a must for this game to remain fresh and not "just another re-release with better graphics for easy money". The fact Valve actually changed much of the maps is another things that sets this one apart from what Source was for the original. Unbalanced maps like Aztec continue unbalanced, but at least are made better with simple additions like boxes and such. Nuke recently got a update proposed by the community in an unofficially released map, now Valve has brought the update into the official map and it got a set of stairs that connects the outside yard with the second bomb site.
Maps that are naturally balanced like Dust2 are kept the same, just visually updated. The first Dust for example also got a set of stairs hat connects the balcony with the underground. Certainly something that makes the map a lot more attractive. If people are just going to ignore the changes, even if for the better, and keep playing 90% of the time in Dust2 with better graphics then it's their problem and their decision. Valve has done what they could. Even if we could use some brand new maps for a change, maybe a map to dethrone Dust2? Who knows.
Two new additions can be mentioned in the guns department. The two grenades, decoy and firebomb. If you're playing the terrorist force you'll have the Molotov Cocktail at your disposal, if you're playing the counter-terrorist side you have the incendiary grenade. Both are the same with different names and models. The decoy grenade emits the sound of the user's primary weapon as a distraction, giving the enemy the illusion that he is firing. Certainly much more suited for competitive players. The firebomb, either version, creates a circle of fire in a determined area inflicting successive damage and slowing the enemy down. Certainly a great addition especially for Counter-Terrorists to use as area denial.
Counter-Strike Global Offensive thankfully remains as Counter-Strike as ever. None of what made the series what it is today are absent on the release. Of course more hardcore players might find something to complain about, stuff as absurd as "the M4A1 Carbine Rifle doesn't feel as powerful as it did in previous versions" but that expected from people who have way too much time on their hands. The truth is, Counter-Strike Global Offensive is a worthy addition to the franchise and for the price tag it's been sold, mere 15 dollars, it's basically a steal. Gabe Newell raises his hand every time someone buys Global Offensive, and you should go ahead and do it.