First time poster, but here is my 2cents. Along with knowing the basic tool set of a level designer, (Unreal, Hammer, etc) you must also have a general knowledge of what is good design. Understanding the needs for a paticular goal and then acting on it. The ability and talent to build a world that supports fun gameplay, style and flow is what is looked for in new people.
A second area of knowledge helps too. Besides just being able to build the level, also knowing how to script combat or script sequences also helps. I know some places have level designers that do nothing but build the world, then others come in and do nothing but scripting. While here at Gearbox all of our level designers do more than just building the world. They also do combat scripting, and scripted sequences. Some also do FX work. We have very talented people here.
As far as getting your foot in the door. Your #1 concern should be your portoflio. This is what's going to get you noticed. If you don't have previous experience, then get some. Many people here came from the mod community. My advice is to get involved with a good mod team, and show nothing but your absolute best. If this is something that you really want, then work as hard as you can to get your porfolio looking as good as you can.
And lastly, there is a reason why on every wanted ad you see the phrase: "Must have a OMG love for video games". You will understand why once you go through your first crunch time. This is an awsome industry, but if you don't love it, then you probably won't last long. It's a very rewarding job, but can be very stressfull at times as well. It's worth it if this is what you love to do.
Hope this helped.
Patrick