(via @sakiamu)
It's more than just battery life. If a running program means I get an hour less work done before looking for a place to plug in, that's not horrible. The experience is the same, just shorter. But power consumption equals heat and that's what really matters to me: if the CPU load in my MacBook cranks up then it gets hot, and that causes the fan to spin up like a jet on the runway, which defeats the purpose of having a nice little notebook that I can bring places. I can't edit music tracks with a roaring fan like that, and it's not something I'd want next to me on the plane or one table over at the coffee shop. Of course it doesn't loudly whine like that most of the time, only when doing something that pushes the system hard.
What matters in 2010 is optimizing for fan noise.
If you're not buying this, take a look at Apple's stats about power consumption and thermal output of iMacs (which, remember, are systems where the CPU and fan are right there on your desk in the same enclosure as the monitor). There's a big difference in power consumption, and corresponding heat generated, between a CPU idling and at max load. That means it's the programs you are running which are directly responsible for both length of battery charge and how loudly the fan spins.
Obvious? Perhaps, but this is something that didn't occur with most popular 8-bit and 16-bit processors, because those chips never idled. They always ran flat-out all the time, even if just in a busy loop waiting for interrupts to hit. With the iMacs, there's a trend toward the difference between idle and max load increasing as the clock speed of the processor increases. The worst case is the early 2009 24-inch iMac: 387.3 BTU/h at idle, 710.3 BTU/h at max load, for a difference of 323 BTU/h. (For comparison, that difference is larger than the entire maximum thermal output of the 20-inch iMac CPU: 298.5 BTU/h.)