Any "tactical" flashlight you buy should be nearly 100% reliable and you should have complete control of it. Depending on your frequency of use, you may want extended runtime on it as well.
That being said, you'd want a tailcap like this:
http://www.surefire.com/Click-On-Lock-out-Tailcap-Z59 . The purpose of that is to be able to turn your light on or off with ease at a moment's notice. Twist tailcaps can be manipulated quickly with enough practice but chances are you'll always be faster with a push tailcap. If you've paired a handgun with a flashlight, you'll know exactly what I am talking about.
Tactical flashlights should be 60+ lumens; I find 80~120 to be just about right for searching rooms and blinding someone. Depending on your task, there are instances where you want 200+ lumens or 40- lumens. But 80~120 is fine for most tasks.
If this is your first tactical flashlight, I would go with a surefire 6P LED defender. It comes with everything mentioned above (80 lumens, 11 hour runtime, push tail cap, and as a bonus, a bezel that can be used as an impact tool). Also, it is a good base for a customized flashlight.
$100 may sound expensive, but many people that buy the regular 6P end up spend more than $100 because they want the tailcap and bezel found on the 6P LED defender.
If you really don't want to spend that much, you can look around for other alternatives such as jetbeam, fenix, streamlight, etc.
Wolfammoman offers great deals often, so visit the buy, sell, and trade section and see if you can find what you want.