I can't speak for all game companies... but I can share some light on what it's like working at Sigil as an associate game designer.
The work we do is very demanding from multiple fronts: creativity, originality, time efficiency, understanding game mechanics and what makes them fun and engaging for multiple play-types, time investment, convincing your friends and family that -- yes -- you are still alive =)
Whether it be the design or implementation of quests, dungeons, classes, crafting, lore, items, abilities, etc, etc, etc -- it is not uncommon to start the work day and realize,
"Hey... it's already been nearly 16 hours and it feels like I just got here!"
Nearly everyday of work has been like that for my peers and myself. It's almost criminal to consider something you love this much as a job... and to collect a paycheck from it =)
For people interested in entering the gaming industry, whether it be as a designer, artist, programmer, or community -- I can only offer the following advice (as I'm relatively new to the working-side of the industry myself):
Complete and total passion for gaming.
Willingness to work insane hours and keep a positive attitude the entire time.
The ability to quantify what it is about the games you enjoy that makes you enjoy them, as well as the things that you dislike about said games (or others).
Creative ability... brainstorming is huge as a designer, and having a number of people in a room all trying to come up with immersive content can be rather difficult -- every single time I've been a part of a meeting like this, someone has come up with something so great that we all stop and say... that's awesome, we're going with that! But it takes a number of ideas off the top of people's heads to get that ball rolling.
Cooperative and "Works Well With Others" (Hello 3rd Grade Report Guard!) -- We work with one another, in various departments, every day... and there are times where arguments emerge... but being able to maintain a level head and professional demeanor, overcoming these types of obstacles is key. Everyone at the company has a passion for their part =)
Stand up for what you believe to be best for the company and the project. If you have an idea that would greatly benefit the game being developed, don't be afraid or shy to voice your idea. Be prepared to go into detail on how it benefits the game, what balances issues need to be overcome, how the game play would work, etc.! We all love crazy and zany ideas, and love them even more if they're very well thought out and don't leave much room to have holes poked into them =)
Experience in the genre you're desiring to be a part of is of great benefit, but not necessarily required. It's always good to know what games are out there now, have been there before, and what makes them tick. This includes actual video games or pen and paper games, of course
Non-defeatest attitude - if something can't be done, whether it be such an "out there" idea or current system limitations, being able to find a way to overcome the issue is a great ability to have. Being able to share (and teach) your methods on how to overcome these limitations to your peers is equally valuable. Many, many times a situation will arise where you ask yourself and others, "What if we could X?" -- and next thing you know, you've found a way to accomplish and bring X to life within the game world!
Overall, though, I cannot personally stress enough how important it is to keep trying. If you've applied to a gaming company in the past and haven't heard back from them, it doesn't simply mean throwing in the towel. There are a lot of people who desire to be involved in game development, and perseverance is definitely a shining quality to have. Most of us have beat our heads against the wall at one point or another
I think it's normal!
I'm not sure how much this helps in answering some of the questions here... as most of this is from personal experience, but I hope it at least provides some assistance to some who read it
Keep playin' those games and if your drive is great enough to join the industry, some development team will surely take notice~