This game is fun. I like the destructible environment and having my arm and leg shot off was a nice touch too I look forward to figuring it out and wasting some time on it.

1) What is the difference between a light soldier and a heavy soldier?

2) Some guns (I'm assuming they are heavy) don't allow me to move. Even the jetpack doesn't take me off the ground. Does this mean I'm basically just an immobile turret with these guns? or is there a better way to move?

3) When I build a base with one of those sliding doors, I can't place my brain behind the door. It gives me the red-dotted line. This basically makes the sliding doors useless to me. Is there a way to open the door during the building phase so I can place my brain deeper in the base?

4) What mode do most of you play? one-man army? skirmish? I'm just curious which modes the community enjoys the most so I can start there.

5) Even on "cake" I die a lot. Either I suck (most likely) or this game is hard. Did you experience this as well?

Thanks for help on any or all of these questions.

The light soldier is weaker, but is cheaper, sexier, and I think has a better jetpack.

How many guns are you carrying? In the current build, certain guns do kinda weigh too much, and the more you have with you, the less you can move. I've whipped up a base.rte mod to increase the power of my jetpacks, if you wanna use it. But I have to warn you, they become really strong. Take the attached .ini file, and place it in Base.rte/Effects/Pyro.

This is currently a bug with the way doors work. When closed, they create a thin layer of terrain, so people can walk on them. It has the unfortunate side-effect of making them broken. One way to work around it, as described by Findude:

Most of the DRL community, as far as I know, plays Skirmish Defense.

The game is just tough at first, but as you learn the nuances of it, it becomes more fun. Stick with it for a while.

1. I'm not certain, but I believe the Light Soldier moves faster and has a longer duration jetpack, and the Heavy has more armor.

2. If your actor has a single heavy weapon and nothing else, it should be able to hover with it's jet enough to get across most gaps. Remember that your weight takes account of all items an actor is holding. This also includes weapon magazines, so if you start to reload a weapon, then switch weapons, the one that you had started to reload weighs slightly less. So yes, you become much less mobile with heavy weapons, so you may want to use a dropship to get the actor to a good firing position and use him like a turret from there.
Finally, note that the game is far from balanced, and this last build severely limited the jetpack power of a few actors, especially the robots 1,2 and the brain robot. The devs have already re-lengthened the duration, but we won't get those changes until the next build is released.

3. viewtopic.php?f=4&t=24987

4. Personally, I don't enjoy micromanaging a large force, so I prefer one man army, although my most-played mission of all time is a variant on that mode called Running Man, available in this mod: viewtopic.php?f=61&t=20903
I would also definitely recommend checking out the community made missions in the scene releases thread, especially those by Weegee or TheLastBanana. Those missions are usually extremely difficult, but see the next answer for my thoughts on that...

5. It is true that the game does have a little bit of a learning curve, i.e. the actors are difficult to maneuver (although a see this as a design decision rather than a flaw in the controls, imagine remote-controlling an entire human body) and the weapons have little to no description attached to them, so at first the game is rendered in to a trial-and-error experiment.
However, even if you have been playing for over two years, as I have, the game remains dreadfully difficult at some parts, but that only makes the satisfaction when you explode an enemy, or even finally beat a mission, even greater.
So, to answer your question, yes you will benefit from more practice, and yes the game is hard, but I think that is just part of the game's charm that has kept me coming back to it for so long.

EDIT: Ninja'd with a few of those answers, but I'll keep them up anyway.