Average gameplay would be enough for me to buy the game because everything else looks good to me. Gameplay is the most important aspect of any game for sure, but everything else around it can still raise a game up a great deal.
Graphics, both in terms of fidelity and art style, look fantastic. The setting also looks great, a steampunk (although I read the devs don't really like calling it that) London, lots of atmosphere.
Some of the music sounds fantastic.
Guns look pretty good, especially that shotgun used in that often-seen gif, as well as the laser beam/arc-gun something or other.
The story sounds interesting, with the half breeds, and the possibilities of what that might bring to gameplay.
My concerns are with the gameplay and length. Insta-fail stealth sections and an overuse of QTEs are a worry. I really don't expect the game to review well (under 70 meta for sure), but I am still hopeful it will appeal to me in the end.