As much as I wanted to like it, it just wasn't fun.
Initially, the plot involves a mad scientist's grandson, the game's protagonist and thus you, visiting his mansion only to find that his brainy grandfather has disappeared into another dimension, yet can still communicate with you through voice-over. As you are stuck through this mansion, you must complete puzzles, test chamber by test chamber, using a device this scientist invented that can manipulate the world around you by 4 different abilities – 2 of them being low gravity and severe gravity. From there comes a great number of physics-based puzzles and platforming.
The actual gameplay has been among the biggest targets of criticism for this game, accused of lacking polish in the platforming elements. While it's certainly not fully polished, the game's FPS gameplay is not as frustrating as some make it out to be. There is a charm to be found, especially in some of the really clever puzzles where you can manipulate the world around you where putting on severe gravity will make everything so heavy you can barely pick things up in this otherwise interactive environment. The opposite ability on the other hand, makes the most metallic objects float from giant fans and is great to stand on, assuming the puzzle requires you to do just that. There are apparently 2 other devices which I have yet to experience though, so I will move on from there.
The biggest problem arises from the lack of coherent motivation, and I am not talking about unlockable rewards; not every game needs them although they are always welcome features. It's the narration and world design is underwhelming almost to the point wanting no return. Your scientist uncle speaks to you a lot from his dimension and while his voice-acting is nothing bad, his dialogue is often weak and intrusive. I know, I'm going with a Portal comparison again, I do apologise. His dialogue and jokes are just weak and in no way on par with GLADoS' charm and hilarity. It's just filled with too much of the cliché 'make things easier for me' and the like. Probably the only time when real humour comes in is when you die and there is text reading one of many things you will miss after death, and even then it's not always something that will arouse more than a few giggles.
I know that even Portal's test chambers were bland in appearance, but that was made up by the game's control accessibility and enjoyment of the actual puzzles. QC is undoubtedly inferior in those departments, and when a game is not exactly up to scratch with gameplay I expect some emphasis on striking environments. While the graphics of the mansion are a lot more colourful than the mainstream "brown" games we have today, the mansion suffers from being overly repetitive in image. In fact, once you have passed the first level you have seen just about all the mansion has to offer in appearance. Even some of the wall paintings repeat themselves along the way. As a result, you will find the most interesting parts come too infrequently when it doesn't take place in a room with furniture.
QC does not suffer from any other flaws beyond the two I have just mentioned, but it's a game that, despite some great puzzles, lacks the ability to be compelling and encourage you to play to the end. I have even been hearing that the game's ending was letdown, but I cannot comment on that. In the end of the day, there is some satisfying challenge and occasional fun and charm to be had when you play through QC, but if you expect too much you will emerge deeply disappointed.