OP, what you are trying to do here is pretend that depth in a game isn't already a fairly well understood term, which it is. Depth for the most part for me means, mechanics that build on one another in interesting and unique ways that allow for a player to implement a unique strategy. For example, the civilization franchise is very deep because, for any one civ there can be many different play styles. For example, the mongols are usually very good at conquering, but that's not the only way they can be played, they can be a peaceful civ too because the game mechanics are set up to allow very many different play styles. Skyrim adds depth by allowing for very customized character builds, with mechanics that build upon one another in ways that may not be obvious on the first play through. Games like tomb raider for me are not really "deep" in game mechanics, each play through will mostly be the same, but that's alright though for a game like tomb raider, because for that game, depth is more about being emotionally invested in the character, and it's not meant to be played over and over. The witcher adds depth by giving the player choices that will have consequences down the line, and can drastically alter a single playthrough
What people mean when they say ED isn't very deep is that, for example, once you've gone mining once, that's pretty much it. there isn't much strategy in mining, just look for minerals and sell them. Or trading, the only strategy I find in trading is finding a route and doing it, there aren't many different ways to go about it. Basically, in ED, once you've tried something, that's it, there aren't many different ways to do something.
Believe it or not though, even the devs here at frontier understand what depth in a game is. The next big update that will allow us to customize our weapons and modules sounds really great to me, and I think will add a lot of depth to the game. Upgrading weapons will add complexity and strategy. Also, once I have my upgraded weapon, I will be more emotionally attached to it than just a simple pulse laser, so it may add some emotional depth as well.
Depth means complexity
Depth means emotional attachment
Depth means allowing for different strategies
Depth means I can do things a little differently each time I try it
Stop pretending like depth is a made up term that no one can really define and is only used by whiners to say the game isn't exactly how they want it. Depth in a game is well understood, and the people at Frontier are working on it.