Great puzzles, interesting new devices and hilarious dialogue.
The puzzles in the sequel are less about how quickly they are executed and more about how they are solved. There is little need for swift reflexes although it helps complete some multi-step puzzles faster. You might be using a turret to protect yourself from fire or lining up a laser beam through portals and switches. Using portals to generate inertia and placing boxes on switches are just the basic brain teasers to get you thinking with portals. Your brain is going to get a workout, not your hands, and learning the new tools helps keep this sequel fresh.
The real key to the success of Portal 2 is the slew of new puzzle devices that are carefully exploited in the numerous puzzle rooms. Initially you will be dealing with the standard box / button / turret mechanics but later you'll deal with hard light beams, excursion funnels (energy beams you can float in) and special gel. Not all the levels are purely test chambers so changes to your thought process are required. Some later areas are very large making things a bit more difficult. When the new devices combine the true genius of the design shines through.
The great new tools at your disposal include hard light, excursion funnels and gel. Hard light is used with portals to create platforms to walk on, barriers that protect you from turrets or block objects from being lost in hazardous goo. Objects float through excursion funnels and they allow you to get some height to enter a portal with speed. There are three types of gel; bouncy, slippery and portal placement. These gels paint the surfaces of chambers, often through portals, and bestow special properties on the area painted. Bouncy and slippery gels propel you forward or upward to generate much needed inertia in confined spaces. They allow you to make a huge leap over a chasm or press a switch located high above. The white gel will let you place a portal anywhere it splashes on the surface although it is not used as naturally as the other gels. With some well design puzzles the solutions are usually very rewarding.
The sequel still has the important "aha" moments although they only occurred for me during the last few puzzles of the single player. There is a slow build up from the opening extremely basic puzzles to the huge open levels toward the end. It should make you respect how far you have come on this journey of scientific discovery. The way Valve has systematically programmed your brain to deal with each puzzle is an impressive feat. The sublime pacing, a key Valve trait, is ever-present and the game will never feel repetitive.
The great dialogue helps push the player forward and even makes the simpler puzzles enjoyable. Very little of the dialogue isn't designed to be funny and it succeeds almost every time. It might be Wheatley discovering he didn't explode when using a light or GLaDOS frequently making fun of your weight. GLaDOS voices the co-op levels too but her dialogue lines are more restricted. When these two distinct characters interact with each other during the story the magic peaks. Great voice delivery and situational humour ensures this game is more than just a series of puzzles rooms.
Co-op is about as long as the single player campaign, which is over twice the length of the original game. In co-op you take up the role of one of two robots, P-body or Atlas, who are destined to save science. They must complete the complex and teamwork focused test chambers. There are five unique chambers, each chamber has 2³ puzzles. Every chamber targets a different aspect you saw during the campaign such as hard light, inertia or special gel. Players can place hints in the world to greatly facilitate teamwork and even time special moves. A new level of enjoyment is reached when you solve the tricky puzzles that employ the intelligent use of four portals.
When first playing co-op the extra portals are used to separate the players so one can stand on a button while the other gets a box. Later the portals are used to leap-frog players through walls they can't directly see from their current location. This teamwork moves up a notch when the robots are separated by force fields that instantly destroy boxes or any portals created by the player passing through. When separated each player has to perform a specific task in their sealed off area. One might send goo through a portal while the other stands on a switch. It is not always as simple as solving the puzzle once. Both robots need to reach the exit and sometimes getting the second robot home is the difficult part.
The experience exceeds the single player during the later co-op levels. A good teamwork example was floating together through excursion funnels while one player protects the duo from turret fire with hard light beams. The extra steps occasionally felt like three people were needed but a few minutes of thought and observation were enough to create a two player solution. Funny moments could be generated by purposely causing death on your robot friend or watching them flounder in one of your portal loops. For a truly great experience it is strongly recommended you play with a friend who hasn't done the puzzles. A new player joining with somebody who has done the puzzles, or replaying with somebody who hasn't is unpleasant.
Portal 2 has very low replay value and repeating co-op levels can even be painful. There is very little enjoyment gained from repeating the campaign puzzles unless you are trying some slightly different solutions. In co-op pretending to play dumb while your friend solves the puzzle is less enjoyable than watching paint dry. If you solve the puzzle quickly you have robbed the other player of the thought process involved that make them rewarding. Providing infrequent hints yields best results for both players but even doing this proves disappointing. The lack of replay value will reduce the long term opportunity for new players to enjoy the same experience as early adopters. The co-op experience is occasionally uneven but you should still appreciate the puzzle design when both players are thinking through each puzzle.
Portal 2 is a great ride, the campaign is brilliantly paced and the puzzles are refreshing and interesting. The co-op makes wonderful use of two players and the last few puzzles in both modes are very impressive. The journey was designed to be completed once but for many this will be more than enough. Free DLC has been promised which should provide even more fantastic puzzle rooms to solve. Everybody should play Portal 2, for science.