“The evidence is generally very weak in demonstrating that the “reality” present in the film is any different than the reality that we experience every day.
1. Repeated lines of dialog shared amongst the characters: Mal and Saito both tell Cobb to take a “leap of faith”, and Cobb predicts what Saito will say in limbo. [However, coincidental and repeated lines are common in countless films without any reason to believe they are "all a dream." Additionally, the term "take a leap of faith" would be basically ubiquitous in a world where one must actually kill themselves just to wake up from a dream.]
4. The clumsiness of the homicide police investigation regarding Mal’s case and put the blame on Cobb entirely. [we don't know all the circumstances surrounding the investigation, and bad investigations happen all the time, so this is hardly evidence for the whole film being a dream]
5. The spinning top at the end of the film. The top is a totem used to help verify whether Cobb’s in reality or in someone else’s dream, but it offers no verification against being in his own dream. He knows the exact weight, composition and how it should spin. So whether the top stops or continue spinning, it’s not important as even when it stops, it could be that Cobb believes that he finally reunite with his family, hence his dream fully becomes his reality. [it's true that if the whole film is a dream, the top wouldn't matter, but this is not evidence for the film being a dream. You could say that about any behavior of any object in the whole movie]
6. In Mombasa, in the bathroom after he tests Yusuf’s sedative, you see the figure of Mal behind the curtains, if he was in “reality” then his subconscious could not be projecting her. [Not projections. These were just memories. The curtains were not in Yusuf's bathroom - they are the same curtains as in the hotel room.]
7. Cobb’s totem was not Mal’s top, rather one could suggest that Mal was his reality check, his ‘real’ totem. Yet throughout the movie he was directly or indirectly responsible for either killing her or imprisoning her, in essence losing his sense of reality and refusing to face up to the facts – which may be that he was indeed dreaming the whole time. [Cobb's totem was Mal's top. In no way was Mal his reality check - it is quite the opposite.]
8. The musical score that is heard, is the slowed down playhead of “Non, Je Ne Regrette Rien”, which is the Edith Piaf song used for the musical countdown. The significance here is that the deeper you go into the dream, time slows, thus the music will slow too, mimicking the score. This musical design perhaps indicates that Cobb’s reality was still very much a dream state too. [the music only slows down when listened to from a lower level, due to the time dilation at that level. Yes, the song was integrated into the score, but only as a theme.] [how do you know? maybe the original poster is right]
9. Numerous improbable or coincidental events that happened during parts of “reality”. Cobb is saved by Saito coincidentally during the chase between Cobol Engineering agents and him. [Saito has been tailing Cobb to protect his investment - not coincidental at all.]
10. Nearly the entire team is highly proficient with all types of weaponry, though certainly Cobb at least, does not appear to have had any particularized weapons training. How does Ariadne know how to even fire a gun, much less hit anything? All appear to be highly capable in all sorts of militaristic tasks, from skiing, to explosives, to hand to hand combat, to sniping… [They are in a dream world during all scenes involving weapon use. Their real world weapon abilities would have no effect here. Like Eames was saying on level 1 'You need to learn to dream bigger' and then produces a grenade launcher. During the dream state, they have limited control over the experience, similar to the Matrix.]
11. This may be a continuity error, but when Arthur collegoes to get Cobb in Tokyo after the failed extraction of Saito, they leave the hotel room and go to the roof for the helicopter at night. When they’re on the roof, it’s day.
More counter-evidence: 1. The children are two years older in the last scene (see cast list).
2. The spinning top starts to wobble just as the movie ends, and also is heard to topple and scoot across the table after it cuts to black.
Alternatively,’ the whole film is a dream’ in the sense that Nolan is suggesting that what we think of as a waking state, or ‘reality’, is in fact just more dreaming (which is not supported by any interviews with Nolan or statements by the cast/crew anywhere).
Hinduism and Buddhism both share the idea that the world is ‘maya’, or an illusion and that we are on a journey of ‘awakening from the dream’. This has been taught by such Indian saints as Sri Ramana Maharishi and Maharishi Mahesh Yogi, who call it ‘Advaida’, which means non-duality; the idea that all the universe is just one consciousness, connecting everything in it. We act out our lives like a play shown onto a movie screen. But only those who achieve Nirvana (a.k.a enlightenment, or becoming one with Nature) can perceive the illusion for what it is (a dream) and become aware of the ‘screen’ on which the great drama is unfolding.
According to the Advaida philosophy, it is only when we are in a state of deep sleep that we come close to the timeless, ‘true’ state of being that is the universal consciousness, which all Nature is part of. When we wake up, in effect we are waking into a world that is illusory. [If I understand correctly, we should be as relaxed as when we sleep, and as aware as when we are awake, in the same time.]
A separate movement believes in the specific idea that humanity is collectively dreaming a dream of guilt and separation, from which we need to wake up by practising forgiveness. This is a metaphysic ideology given in A Course in Miracles and The Way of Mastery, which are spiritual texts purportedly channelled from Jesus in the 1970s and 1990s respectively. ACIM in particular has influenced most spiritual ‘new age’ authors and commentators today.”