My Research Methods class has been wrestling lately with the relationships between statistical significance, power, and effect sizes, and with the balance between what's best, or most true, and what's actually practiced by researchers.
This post from a few months ago has some nice discussion of the difference between small-but-significant effects, and big whopping useful/meaningful effects:
The problem with small effect sizes is that they mean all you've done is nudge the system. The embodied nervous system is exquisitely sensitive to variations in the flow of information it is interacting with, and it's not clear to me that merely nudging such a system is all that great an achievement. What's really impressive is when you properly break it – If you can alter the information in a task and simply make it so that the task becomes impossible for an organism, then you have found something that the system considers really important. The reverse is also true, of course – if you find the right way to present the information the system needs, then performance should become trivially easy.
Their example of breaking the right thing is a bit hard to understand without reading the linked materials, but their example of fixing the right thing is beautiful:
A real problem in visually guided action is the accurate, metric perception of size (to pick an object up, you need to scale your hand to the right size ahead of time). Study after study after study has showed that vision simply can't provide this without haptic feedback from touching the object; but we do scale our hands correctly! The question is how do we do it? Geoff has been plugging away at this for years, trying to provide people with what he thought were sensible opportunities to explore objects visually, with no luck, until he rotated the objects through 45° (a huge amount in vision). BAM! Suddenly people could visually perceive metric shape, and this persisted over time without being constantly topped up (Lee & Bingham, 2010). Suddenly we knew how we did this task; metric visual perception of shape is enabled by all the large scale locomotion we get up to – moving into a room, for example. Without this calibration, the task was impossible, but as soon as the right manipulation was made, the impossible became straight-forward, and the effect size is huge.
If you're still trying to make sense of why psychologists care about effect sizes, take a look.