I've been thinking about what's been fashionable, or what has been fashionable in various areas of game development. I've drawn no solid conclusions, except that fashion is something which is perhaps overlooked a lot of the time by developers and people who play games (I hesitate to use the word "gamers", since that in itself has become unfashionable). Off the top of my head, I can think of a lot of aspects of games which became fashionable, enjoyed a period of popularity, and then fell out of favour somewhat.
- Point & Click adventure games. Became cool in the early-to-mid 90s as a more colourful extension of text adventure games. Fell out of favour when it was widely realised that one overly annoying puzzle could completely halt progress in a game and that the only fix was a walkthrough, which somewhat detracts from the experience. The last great adventures were Grim Fandango and The Longest Journey, right at the end of the '90s. They're enjoying a bit of a comeback now due to the works of Telltale Games, but they feel a bit watered-down in their recent incarnations.
- Lens Flare. I forget when this was first introduced, but in my mind it's cemented as being a hallmark of Sega games (and other vendors of Blue-Sky games) in the mid '90s, as a way of emulating the effects of sunlight on a real-life camera. Graphical tricks have a history of being incredibly cool at the time, and then getting old fast: parallax scrolling, rotoscoped animation, particle systems, bump mapping, HDR lighting, depth of field... All of these things look like wonderful innovations at the time, but get tired quickly.
- Physics. Games have had physics since Spacewar, but for a while having a particularly realistic physics engine was a selling point. It was happening before, but for me, Half Life 2 was the first thing that really made my jaw hit the floor. Now, realistic physics are practically expected in many genres even if they add little to the gameplay, and a game which advertises its physics as a selling point is generally very focussed on the physics and little else (for instance, the Jenga-esque joy of Boom Blox). Physics is still hot stuff right now though, so long as you're willing to sufficiently raise the stakes -which is part of the reason we're doing fluid physics. We feel half-annoyed, half-vindicated by the fact that the makers of PixelJunk Shooter feel the same way.
I suppose it's a good thing for an indie to try to tap into an upcoming fashion in order to get recognition. Braid wasn't the first game to feature time manipulation, but it was early enough to get recognised for it - perhaps in a few years everyone will have jumped onto the time manipulation bandwagon and it'll be an old, tired concept. Something like Crayon Physics, on the other hand, took long enough to be released that although it's a brilliant game, it felt a bit "old" on release, because of all of the clones that shamelessly preceded it, and because games in general have begun to take physics plaything for granted. On the one hand, indie games have the flexibility and bravery to enter uncharted (or at least under-explored) new territories in gameplay and innovation, and profit from mapping them; on the other hand, indie developers might not have the means to get a game to market before today's New Cool Thing(tm) becomes tomorrow's Old News.