The Metal Gear Solid franchise has always prided itself on evolving with the times. Each new entry isn't just a prettier version of its predecessor. In the four core entries in the series, each feels as unique as the others, whilst at the same time maintaining that special Metal Gear feel to it we have loved. And all of them are brilliant.
MGS4 represents for many the perfected version of "Tactical Espionage" Hideo Kojima set out to create over 20 years ago. The camo system speeds up the concept introduced in MGS3. The overdue camera change allows for a better view of the field. CQC has been improved. Customisation of weapons is so advanced that it allows so many more options is how you wish to approach the game. Herein lies the biggest factor for MGS4. Previously being seen wasn't really an option. In MGS4, it is a viable option if you choose. This adds replayability as it means you can play the game as a stealth game, or you could play it as an all out gun affair, and both feel like they belong.
The concept of a 5 chapter world trotting adventure surprisingly suits Metal Gear. Each chapter contains a sneaking section, a special event and a boss fight. On paper this seems simplistic, but in its implementation it creates so many incredible moments that when you finish, there won't be just one memorable moments, there will be loads. Each section is unique, one sneaking section has you trying to make ground in the midst of an ongoing battle. Another Sneaking section has you tailing a suspect in the middle of a curfew. The boss fights are both challenging and enjoyable, a part of MGS which had previously been its weakest. There is no sense of repetition in MGS4. Every section has its own identity, and every section is both enjoyable and memorable in its own right.
For those who dislike the overuse of cutscenes in Metal Gear, you'll be unhappy to learn that MGS4 takes it to ridiculous proportions to the point you are asked to save in the middle of one, because it goes on so long. I have always loved the MGS story and for me many of the special scenes in gameplay (a certain split screen section) just wouldn't have had the same effect if you skipped the cutscenes.
I can't speak for those who dislike the story, but for me this game represents in some respects both one of the best games I have ever played, and one of the best movies I have ever watched. Many games these days (The Uncharted series) have that sort of feel to them. But most of those seem to have less appeal once you completed the game for the first time. Metal Gear Solid 4 has been coming back to it again and again, each time wanting to try something different in how I play it.