What is your most important pro Bitcoin argument?
Bitcoin was designed, where there was no such thing as "cryptocurrency". There were only fiat currencies. In 2009, people didn't even know, what "altcoin" means. And because of that, the design assumed, that it is the only coin, made of bits, and there is no competitor, besides fiat, and centralized banking services.
Which means, that if you want to create a successful altcoin, you have to assume, that Bitcoin is already there. And then, you have to compete with it. You have to provide something, which can be done only on your chain, and nowhere else, and what cannot be copy-pasted into Bitcoin, because it would be considered "out of scope for the payment system".
In general, if you want to make an altcoin, then it should not be focused on producing any new coins at all. If you do so, then you repeat something, which is built into Bitcoin, and you have to compete on the same field, which is difficult, and most altcoins die, when they try it (another reason not to do that, is because then you abuse 21 million coins limit, by creating more of them, out of thin air).
A good example is NameCoin: imagine how better it could be, if it could only produce new domain names, without producing any new coins at all, and if all NameCoin transactions would be Bitcoin-denominated, so that you would have 1:1 peg with Bitcoin, and use them for all of your purchases. Also imagine, how good NameCoin could be, if there would be a market of vanity miners, which would mine you some Bitcoin addresses of any type you want, and would receive some domain names (or some Bitcoins, not NameCoins) as a reward.
My most important Bitcoin argument is PoW
The only reason why a lot of altcoins switched from double SHA-256 into something else, is because they still don't know, how to implement Merged Mining correctly. In general, what they should do, is to always keep track of the heaviest chain of 80-byte block headers, no matter, if they are valid or not. Then, based on that, they should calculate the global double SHA-256 difficulty. And then, they should allocate any actions, like producing some new coins, in a way, which would be related into that.
For example: if Bitcoin can reward someone with 4 BTC, for difficulty X, and your altcoin has 1/1000000 of the total double SHA-256 power, then your altcoin should reward your miners with an equivalent of 400 satoshis, and not for example 50 ALTs. In the best case, it would be related to your own product, like "a new domain name", and not "just some dummy number, worth 400 sats".
