This text may contain traces of other Abrahamic and Lovecraftian deities. Research results by +Danna Staaf, Cthulhuckick and +Tim Minchin are used herein.
1. Holy books: Christians have holy scriptures about their non-existing god. We even have non-existing holy scriptures.
2. Sex: No nonsense about virginity, no taboos. Fornicate as much as you want. Of course, you must use your third right tentacle for that, anything else is an abomination.
3. Food: No taboos, no fasting, no trouble finding out what is kosher/haram/halal/trefe. Essentially everything is allowed (Calamari is verboten, though).
4. Free speech: You won't get stoned for pronouncing god's name (but your friends will say "Gesundheit" and offer you a handkerchief if you do).
5. Freedom and Privacy: No divine surveillance, no celestial North Korea, no confession. You can do what you want - our god sleeps anyway.
6. Eucharist: You eat pieces of your god and drink his blood (if you are catholic) - our god eats you.
7. Maths: Christians burn mathematicians and their books since 1600 years. Cthulhu wants you to learn non-Euklidean geometry, which is fun. Enroll for a course at Miskatonic University now.
8. Technology: Cthulhu's creatures (Ommastrephidae) had rocket science[DS, PhD thesis] when the other one's beasts still lived in caves or even on trees.
9. God's voice: Nobody ever heard the voice of the biblical god. Cthulhu spoke and was recorded by hydrophones.
10. Justice: If you were sent to hell unjustly, you've lost. If you were eaten unjustly by Cthulhu, you can appeal to Nodens.
11. Priests: The pope sucks. PZMyers rules.
12. Music: Christian songs have nice chords but the lyrics are creepy [TM, "White wine in the sun"]. Azathoth can play creepy chords on his flutes.
13. Kittehs: Holy creatures in and around Ulthar. In Christianity they are considered camouflaged witches or the devil's messengers. I have to admit that Mohammed fixed this.
14. Tentacles: Even the lowliest, dumbest things on the seafloor (corals) have them, and so do the nasty evil Box jellies. And you worship a "god" that lacks them? (Hinduism is slightly better - Ganesh has at least one tentacle, and Kali tries to compensate by having many arms). This problem has become worse over history:
Quran V1.0: "Have you thought about tentacles, suckers, and hectocotyli? They are the divine attributes, and their presence is mandatory".
Then the rulers of Mekka looked into a mirror and saw that their faces lacked something, filed a bug report and asked the prophet for a patch (that totally reversed the message). The result became:
Quran V1.1 (i.e. the shipped version): "Shall we have beards and he only tentacles? That would be insane!".
15. Copyright: The theological works of the first prophet HPL are available for free download. Bible versions which fix at least the gravest translation errors (e.g. RSV) are copyrighted, you need wget and scripting to download them.
16. The apostles followed Jesus on earth. Cthulhu follows me on twitter.