i think that grafics always have to support gameplay and not the inverse way. having a good gameplay is the base of a good game, for example Dwarf Fortress is and exelent game with almost no art, just some pixels for displaying waht is going on, but, it's always a good idea to put some dedication in the "look" part because the first impression always comes through the eyes.
Looks are required to draw the attention. Gameplay helps to keep it, but most of all it has to fire a player's imagination - whether they live vicariously through the eyes of a weenie turned supersoldier or are somewhere so weird that it invites exploration.
That third part is the most important. Books, movies, films, music, games - the ones that people go back to are the ones that capture their imagination.
Capturing the imagination can be as simple as appealing to the right hormones for the right age group, so it's actually not a necessity for the game itself to be original. In fact, if the player has to spend a lot of time familiarizing themselves with it, it destroys the immersion.
This is why "me-too" FPS games and warrior hotties swinging swords as big as cars and wearing chainmail bikinis continue to dominate the shelves despite an utter lack of anything new or cool about them in the last ten years...
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Many comments here are interesting (i've not read all of them !). Sure many people speaking here are really game lovers and it's good ;-p ... a game lover say GAMEPLAY before !!!!! in fact it's quite normal as poor gameplay games go too soon to trash to stay in memory ;-p
Html5 games creation is quite different for classic games. When creating a classic game graphics are really the most important "first" for 90% of games ... and people waiting for a new game only seeing 3 screenshoots can't say it's not true !! .... so a game is unplayable in early state of creation unless using last computers on market and last high cost graphics cards and so on ... but as this game will be finished in 1 or 2 years ... sometimes more ... it should be will be very playable when published ;-p
For a html5 game which can be created in a couple of days/months is really different. As acord say graphics are important to bring first interest in game ... but obviously a game which can make you spend hours and days in it can't be "only" a beautiful game and so for that gameplay is needed (not only fps but also game "thinking" for players pleasure when playing).
So we have game thinking for looking pleasure and game thinking for playing pleasure ...
Some games "creator" have made the choice to "sell" games with poor gameplay a so very short life but with very good graphics to bring interest until going to trash very soon ... for me it's more games sellers than games creators but it could work too as the players profile deeply change in a couple of years ... hard core gaming is quite over ... so quick played and dir... beautiful games ;-p can find players too ;-p
The best stay graphics and gameplay, if not possible it is the way you want this game will be played and targetting players profile which help you to find the better choice.