Hey! I've got some engineering concepts for you about batteries!
Yes, I'm an engineer...
And to tell you the truth, I'm quite appalled with everyone's ignorance when it comes to understanding and predicting battery life.
First, I will say that there is a difference between maximum usage and typical usage of a computing device, whether that be a desktop, laptop, or even a smartphone. I'm just going to give you some hard engineering numbers and facts to help you to understand why computers (and tablets in particular) get the battery life they do.
So, I've organized a few major points below for your consideration.
1. Energy can not be created or destroyed. (1st Law of Thermodynamics)
To be blunt - this is why so many people are over-weight in the United States. When someone consumes calories, they are effectively adding energy to their body. When they don't expend the energy, it becomes stored as fat. The only reliable way to loose weight is to consume (eat) less energy than you use. You might be asking yourself what this has to do with batteries - well, I'll tell you.
The human body can be metaphor for a battery. A battery can consume and use energy. The main difference between a battery and a human is that a battery has a very limited amount of usable energy. The average human male will consume 2500 Cal in 1 day. In terms of battery-lingo, this is 2910 Watt-hours (Wh). Just to give you a picture here, the 11'' Macbook Air has only 35 Wh. The MBA's battery is about 1.2% of the calories a human will consume in a day. A larger battery in a laptop may have about 5 times that, at about 6%.
Technology can not keep up with battery improvement as fast as it can processor improvement. From about 1990, battery technology has only improved by a factor of 3. This is compared to an exponential growth of processors. Battery technology is lagging far behind.
2. Rate of Energy Usage (dU/dt)
Energy is consumed at a rate. It's a fairly simple concept - think of gas in your car. When you're going 90 mph on the freeway, you're going to run out of gas sooner than you would if you were going 60 mph. (be careful not to think of distances traveled here) You have to use more gas to go faster. The rate at which something is consumed is a variable of time. In engineering, we use calculus and call it a derivative (dU/dt). Believe it or not, the standard unit of energy consumption is the Watt (W). It's a common term, but I will say I want to hit my head against the wall every time I hear someone say "Watts per hour". A Watt already takes time into account. For the more technical, it's rated as Joules/Second.
So, in conjunction with a battery - if you're using more energy, your battery will run out of 'gas' sooner. Who has heard of Intel's Ultrabook 17W TDP processors? The 17W is actually the rating of maximum energy usage. Just to give you a number here. Again, let's take the MBA as an example.
The 11'' MBA uses a 17W TDP processor. A good estimate would be to add 3 W for the screen's energy usage, and maybe 1 W for other processes such as WiFi. That brings our maximum energy usage to 21 W. Remember, this is maximum use. So how long would that last with the MBA's 35 Wh battery? Well, think - 35 Wh / 21 W = ? Try 1 hour and 40 minutes.
You may be saying to yourself that the conclusion is wrong. It's not. What you should remember, however, is that we're looking at maximum usage. Lets try something more moderate - say browsing the web.
Let's say we use about 15% of the processor's power (web browsing) with the same processes as above. 2.5 W for the processor, 3 W for the screen, and 1 W for WiFi = 6.5 W. Using the same approach as before: 35Wh / 6.5 W = 5 hours and 20 minutes.
3. Processor Architecture is Fundamentally Different
Specifically ARM vs. X86. ARM processors found in the iPad and Surface RT are very different than higher-performance processors. Without going into the technical details, ARM technology is fundamentally less power-demanding than X86 processors. Lets just take a look at the iPad 4's processor vs. the Surface Pro's in a maximum performance showdown. (Numbers from AnAndTech)
Processor/GPU maximum for the Surface Pro: 17W TDP
Processor/GPU Maximum for IPad 4: 4W TDP
Performance of Surface Pro: 879
Performance of IPad 4: 182
Performance per Watt:
TDP/Performance Surface Pro: 51.7
TDP/Performance IPad 4: 45.5
As you can see, the performance per watt is roughly the same on the two devices. The Surface's i5 actually manages to beat out the iPad's A6X. You should just keep in mind that Intel has a huge manufacturing advantage with their cores that allows them to overcome the advantage of ARM processors.
So, what does it matter to you - the end user? Well, how much bang are you getting for your buck? Literally and figuratively.
iOS is a light operating system that requires minimal processor function to run. Conversely, it's capabilities are also limited. Windows 8 is a heavy operating system that requires a larger amount of the processor to run it, but allows some heavy-lifting operations as well. This is well demonstrated with the Surface RT. The operating system chews up more of the processor just trying to run Windows 8 than iOS. Because of this, they needed to reduce the energy usage from the screen - resulting in a lower resolution.
Ultimately, how would you spend your processor and battery? It's not unlike the number of years you have in life. With current technology, the capabilities are limited. Would you rather have a hard limit on processing power with longer battery life, or the full flood of an i5 processor? Would you rather have a lighter device that can't last long, or a heavy device that will give you hours and hours of use?
The market has all of these options out for you at this time. It's yours to choose.