Yes, the visual and unique flare are key.
Unless people receive great impressions from someone they trust, when they see screenshots or a trailer with bad visuals of a game they just found and is from an unknown developer, they quickly avoid it.
Regarding uniqueness, I (dev with almost 20 years of experience working in over 40 published games, some of them award winning or top in their market/niche) think that the most important thing is to have let's say 80%-90% of the game based on proven and solid grounds, based of several different games that worked really well in this genre/target user.
And then the rest being something different or not common that helps it to standout from the rest. Doesn't need to be new or unique, maybe a mixture of popular things others didn't do before, or going an extra mile on certain area, or doing an interesting twist to a known concept.
The most important thing, more than being unique, is to stand out from the similar ones, to provide something that makes it more appealing. As could be an aesthetic or character design that players prefer, to have better visuals or performance, or something else that makes it feel fun or different. Which could be to add a new concept, mechanic, theme, etc.
But the most important thing is to make sure that when your target user sees the game thinks it looks and feels great. Regarding hack & slash / 3D beat 'em up, in many cases I see new ones in Steam and quickly skip them because they look ugly to me, or I see that the animation or speed feels really bad and I know the game will be boring. Often also because the visual and audio feedback of the hits sucks, when it's something key for action games.