I got into an argument with a fellow member on here, he claims that a certain unreleased game is 100% gauranteed to be the best in the series. He came to this conclusion based off what the developers have said about the game, a few trailers and the because the game introduces new features/characters and has better graphics.

On paper that indeed sounds promising but there are too many unknown variables that could cause the game to not live up to its potential. Im sure most of u have seen games that looked awesome but once they actually released the game sucked. Of course a developer is going to clain a game is great no matter how shitty it actually is and an awesome trailer definitely doesnt mean the full product will be equally as great. Things like framerate issues, game breaking glitches/bugs, unresponsive controls, horrible camera angle, bad story, bad character design, repetition and countless other things can cause a game to not be great.

What do u guys think, is it possible to know a product will be good/bad if it hasnt actually been released yet?

"I think most self-described "hardcore gamers" live out the hobby vicariously. Outside of a handful of titles, they talk authoritatively about experiences with games they've never played as though it was firsthand experience with words stolen from the opinions of others.

This embodies most of the much-lauded “community” in gaming as a whole:
People talking about things they often haven’t experienced, trying to impress people they don’t know, about a topic that rarely matters outside of an individual’s experience. " - Words of wisdom found on the internetz. Yes it happens. 

Your taste and his taste varies. So you guys are fighting over opinions without much conjecture. 

Nope, you can only assume it will be "good" by cool trailers or by who's developing it but there is absolutely no way to determine a game is "good" without playing it.

It's called 'Gamer ESP'

Just like how people that saw the Burton Batmans knew the more expensive Schumacher ones would be better. Must better. (Snarf) Just kidding, I kid, I'm kidding.

Nah, seriously, why you arguing over something that fiercely that neither has played? You should find a game you both play and try to out do each other for bragging rights.

I agree with everything u said except its not about our idividual tastes because I think the game looks fantastic as well, its just him claiming to know it will be great. If he would have said "this game looks like it could be the best in the series" then I would have agreed.

nah. Square Enix made FFXIII looks amazing in the trailers, and based on the series' past, and this huge graphical step forward, we all assumed it would be the best in the series. it turned out to be the exact opposite of that....

Games are not a mathematics. One can not simply add up the numbers and figure out what makes a game great. Certain elements need to in place for a game to be great but even with all those elements in place a game's ultimate quality can still fall anywhere on a very wide spectrum.

A developer or gamer may see that a sequel has more of the 'good' stuff and less of the 'bad' stuff than its predecessor but that is far from a guarantee that the sequel will actually be objectively better.

Idk I just have a hard time not calling people out when they say ignorant things and to me saying a game is undeniably going to be great before it releases is ignorant. Its just as bad as saying a game is terrible when it hasnt released yet.

Technically, no. There's lots of ways to be 99% sure, but never 100% until you physically play the game fully and coon confirm one way or the other.

It's absolutely possible. You can use developer's past history and credibility for making games to predict the quality of their next error. If there are any red flags that come up in previews, then you can change that opinion.

I'm certain that the next Persona is going to be fantastic because there's yet to be a bad Persona game. I'm willing to bet that FF15 will also be great because of who is working on it. You obviously can't necessarily review something based on watching it, but there's a reason why I ignore most reviews and just buy what I'm interested in. It works for me far better than reading what journalists have to say.