1. It's extremely unique - I assumed from the trailer that it was sort of similar to Liar,Liar or Yes Man, however it is actually very different. The film paints a completely "new" world for the audience - a world where no one can tell a lie (it's not possible and never was...until now) which opens up a whole new world of possibilities. It asks the question "What would the world be like"?....which brings me to #2.
2. It's about religion (from an obviously Atheistic point of view)!! - Obviously they didn't advertise the religious context of the film for good reason (i.e the over zealous religious people who would throw a fit). The film doesn't shy away from the biggest question of all - "What would the world be like if there were no religion?" (Because all religion is just a bunch of lies...get it?). The answer is that the world would actually be pretty dull and depressing if there wasn't something to believe in...such as an "after-life". So, the film doesn't really say anything bad about religion per se...just that it is not realistic (or scientific...or the truth..). I love that Ricky Gervais is an Atheist and made a film about Atheism and masked it as a romantic comedy...it's absolutely brilliant.
3. The beginning is really funny - The novelty of the film is that it basically makes fun of everyone for being so gullible and easily persuaded. It works really well for the first 40 minutes, but then the novelty wears off and the audience laughter became less and less. It also becomes a little repetitive - we get it Jennifer Garner is out of Ricky's "league" because he is not her "genetic match" and she doesn't want "little fat kids with pudgy noses". I would still recommend it though...for it's unique story alone.
4. Surprise Cameos - I won't say who (otherwise it would ruin the "surprise"!)