There are many variables that would make one proficient at being a level designer. Obvious ones would be for starters: Computer literacy. Phil Co (A level designer who IIRC has worked on some Valve content if only as an out-source, amongst many other projects) has several informative books out on the subject. In one he lists 8-10 fields of expertise to be versed in to give you a good foundation in proficient level designing. Ranging from Physics and Geology to Art, Architecture and Design and even Psychology. But generally speaking a good knowledge of how the world operates is what is expected. Because if you can't understand how the world works, you can't hope to replicate it in any relatable way; and your whole goal as a level designer is to maintain a balance between fun game play through the geometric design and the suspension of disbelief through its aesthetics.
Some games you can get away with not being versed in certain fields like physics and geology because the universe is so abstract the same rules that apply to us, don't here and can thus be largely ignored. However in a game like Counter-Strike structures need to look like they are supported like they would be in real life and to be able to effectively and reliably demonstrate this is really the keystone to the entire operation.
I would say if you were versed in most of these subjects, had good 3D awareness/comprehension and of course basic computer literacy i would say you would have the talent to become a level designer without any real training in it. Though training would be ideal. You'll probably find most of the successful people here, and further--those in the industry, are well educated, particularly in these areas.