Art is the hardest part because Art is a skill you are either born with or cultivated when you were wrong. (You however can design, edit and create without the skill)
Coding however is something you can learn easily.
Art also as you have listed is more time consuming. One of the problems with indie game development like i said is #1 every one wants to do things solo (whether its because they want to stick to their idea or not) and then #2 people working on game logic instead of Art first.
That is a huge problem. Its hard to stay motivated as have been said by @vfez when all you have is game logic and nothing else done.
Art should always be #1 focus.
Its what people see first.
Think about the first 60 seconds of the new Metro trailer. If someone took a picture of that and posted it here people would go crazy and want that game, it will garner alot of support and solve the motivation issue.
Art trumps everything because its what people see. Yet people don’t get that they focus months on game logic that is barely reuseable than quit or complain or simply buy cheap assets from the marketplace and throw them into their environment without no texture retouching and you end up with your run of the mill marketplace game.
