As bike lights get more and more sophisticated the switch does more than just turn on and off your lights. It also tends to allow you to select one of what may be several different modes. That means that the switch cannot be fiddly to operate. You might after all be out on the trail in the middle of Winter wearing thick gloves and you need to still be able to operate your lights without having to stop and take them off. The flip side of this is that they should also not be easy to operate accidentally. A lot of manufacturers have now started to add backlit switches to allow you to see where the switch is easily at night. Many of these also double as battery life indicators as well as mode indicators.
There has been a revolution going on in the battery industry and ever more efficient, charge resistant Lithium Ion batteries are still evolving. Current batteries are not only more powerful but they are also lighter and tougher than they have ever been before, especially when compared to older lead acid and NiMH batteries. However, as I said, things are still evolving and the efficiency of batteries can vary wildly from one model to another. So, when you are out shopping and comparing different lights don't try and estimate run time by the size of the battery and quoted ampere hours of the manufacturer. Seek out independent reviews in your favourite cycling magazine or online and go by the tested run times.
There are many different ways now to mount a light to your bike and in some cases you may not want to mount the light on your bike at all and may want to opt for a helmet mounted light, or maybe even both. Go for the type of mount that suits you best and this will depend on the main type of cycling you do. If you go for a helmet mounted light though, make sure that the mount is really secure but also ensure that the total weight of the helmet once the light is fitted is not to high or you will end up with neck ache.
This is probably the item that you will least be concerned with when buying new lights, but that may be a mistake. There are many charger types to choose from, including simple USB lead affairs to full on battery docking stations that let you set up your lighting options via your PC. Again, choose something that suits you and remember to check what the quoted charging times are or you might end up being left in the dark if you have chosen one that takes a long time to charge when you only remember an hour before you need your lights that they are flat from the night before.
In a similar way to the battery industry, the lighting industry has also been going through a bit of a transformation recently. It's not that long ago that LEDs or Light Emitting Diodes were not practical for anything more that decorating the Christmas tree due to their low power. That has all changed though and year after year LEDs are emitting more light. For example, on average, cycle lights that are on offer in 2012 are 7% - 10% brighter than they were in 2011 and that trend is set to continue. They also draw a lot less power than normal bulbs and therefore your batteries will last longer. One thing to be careful of though and something you may want to compare between one light and another is the operating temperature. Some of them get quite hot and controlling the heat they produce has been one of the challenges for LED manufacturers.
All in one systems for lights are becoming increasingly popular as they do away with the need for leads running from the battery unit to the light unit. However, being aware of the limitations of two unit systems is important. Cheaper systems may only offer simple 'water-proof' jack to jack leads but the more you pay the more you tend to find an efficiency increase in complexity and sealing making your lighting system more reliable, especially in wet weather.