-Do you have a preferred art style
Non cell-shaded. High concept in general. I prefer grittier, darker presentations to really high color/high contrast environments. Just gets tiring on the eyes after a while.
-How long should the game take to play?
20 hours for the "basic" experience is usually my bench mark. Any less than that and I feel like I should be paying for a cheapy indie game.
40+ hours at least for "the entire package", secrets, different endings, upgrade schemes.
100+ hours for a MP or co-op game before I get sick of it, or for truly epic games that offer tons of ways to play. (Dwarf Fortress as an exemplar.)
-What aspects of the game do you like to have control over
Great question.
Identities, first and foremost. Names are a must, I hate playing games with many controllable characters, but I'm forced to accept the names they are given and their personalities because they're part of some grand story. The later Final Fantasy games as an example.
Save points. I hate games where you have to commit to something from start to finish in one sitting if you want to do it. After the initial love affair with a game, if I know I have to commit many hours to just one sit down experience to get enjoyment out of it, I tend to stop playing it. Being able to pick it up and put it down when I want is very important. That's why games with "Mission" structures tend to annoy me, because you are forced to commit more and more time as the levels get tougher.
Flavor. Color options on your characters (like Alucard's cloak from SotN), settings where you can design and/or decorate them to taste, stuff like that. Anything that makes me feel like I own the choices I'm seeing in game.
How important is challenge?
Important? This is a tough one to answer because difficulty is subjective. And there are people who don't like to be overly challenged in their video games, or there's a setting that doesn't really lend itself to being challenging.
In general....I enjoy my experience more if I know there was a decent margin for failure based on skill. I DISLIKE experiences that are so hard they are meant to be replayed dozens of times until you learn how not to fail. I DISLIKE failure as a random chance..i.e. a random monster in an RPG with instant death spells. I ENJOY challenges based on random chance, but not ones that are usually guaranteed failure.
On the other hand, I enjoy stuff less if I know the only way you can fail is gross incompetence, where your experience is almost entirely a gimmie. Assassin's Creed, Devil May Cry, a lot of 3rd person fighting games fall into this trap at times, where they get so focused on making sure you look cool beating people up and have lots of crazy combos to do....that they forget to make it challenging. I still enjoy that more than near guaranteed failure.
Whatever you feel is important
Too broad really. I guess what I'd always caution game designers about is never assume that a single idea has enough strength to ride on its own. Take for example, "a game where you collect gear that makes your stats better and junk." That's relying on tropes which, to experienced gamers, are dead ends for entertainment. We know those systems, we've lived those systems, and there's no joy of discovery in just slotting in what has worked before. Always think about ideas as they relate to the whole game, and look for ways to connect as many aspects of game play together. Discovering those connections and teasing apart how the game works is where I get the most joy. Games where all the features are essentially modules that are dropped into a framework (FPS, 3rd person, top down, 2d, whatever) aren't dynamic or engrossing.