1. A general purpose handheld tactical light should put out between 60 and 80 lumens
This is another point of controversy in the world of flashlight geeks. A lot of people are, to borrow Ken Hackathorn's description, "absolutely queer" for how many lumens a particular light can generate.
Contrary to what people think a light that can double as the bat signal or that puts out enough light to melt the face of a bad guy like that Nazi in Raiders of the Lost Ark is not necessarily a good thing. Remember the opening discussion about the rods in our eyes and their saturation point? The more lumens your light puts out the higher the chances that once you use the light you will actually "flashbulb" yourself when using it.
Most interior structures in the United States have walls painted with light colors. Light colors reflect more light than dark colors. Somebody who lights up a normal residential hallway with a 200 lumen face-melter even for just a split second is going to experience what is known as the "flashbulb effect". (Be aware that “lumen” measures are often wildly overstated by some companies)
The flashbulb effect happens when the rods in your eyes are hit with a flash of light sufficient to saturate them, but not long enough to allow the cones in your eyes to take over completely. This leaves you literally blind for a short period of time... completely unable to see a bloody thing. You can ask the guys who went through the Vickers/Hackathorn low light training courses about too much light.
If anyone doubts the validity of the flashbulb effect, conduct a simple experiment. Find a friend who is a photography nerd and ask to borrow their big camera flash. Wait until it is completely dark in your house and give yourself at least 30 minutes in the dark for your eyes to adjust to their peak night vision efficiency. Then point the camera flash down the hallway and trigger it.
See how long it takes you to recover the ability to see to a useful level. You will then understand why it is called the "flashbulb" effect. (Note: You can also set up a game camera in the bathroom and ask the missus to report back on what effect it had on her during the night, but be sure you have a comfortable couch first.) A 60-80 lumen light offers enough power to clearly identify what is going on within the hotspot of the light, but not so much light that you obliterate your night vision entirely.
2. LED lamp elements vs incandescents
LED's have proven to allow for longer battery life and can survive more abuse than incandescent lamp units. The downside to LED units is that they don't tend to penetrate smoke as well as incandescent lights do.
3. The activation switch should be pressure based
...meaning you have to apply deliberate pressure to keep the light on rather than a "click" style on/off switch reminiscent of the old Mag lites.
The pressure based switches are a pain in the neck if you are using the light to fix a circuit breaker or a fan belt... but click style switches are even MORE annoying when you are trying to use the light in a tactically sound fashion or in conjunction with a handgun. Under stress they inevitably get switched on when they shouldn't or left on for far longer than is desirable, making the person holding the light a target.
There are a number of switch styles on the market that do everything from just keeping the light on with a click to engaging a strobe function... but my advice is to avoid anything that isn't pressure based. The simple lock-out tailcaps seen as standard equipment on the Surefires pictured here and that are the default on most Surefire lights are already very good for their intended purpose.