I think it is good that we're taking a step back and examining where each of us is headed with our various lists. The tighter the focus of a list, the better it showcase the particular aspect of the Mechanicum at War that we're trying to portray.
Also, tighter lists are easier to playtest (fewer variables means less tests to get satisfactory coverage) and harder to abuse (there are fewer odd or unintentional combinations that the designer and the testers miss).
With that said, I'd also advocate keeping focus on making a balanced, NetEA Approved, Tournament ready list, from which the variants can draw on unit stats and common formations. I'll promise that this is where I'm focusing my games, until such time as the list is deemed done.
With that out of the way, I'd like to try to summarize lists and their missions, as I see them:
The theme, then, of the core
Adeptus Mechanicus PDF list, is the Skitarii augmented soldiers (with Servitor and Robot auxilia), and the weird and wonderful Mechanicus Ordinati, with a secondary focus on the many kinds of Titans that support them. It should play well both offensively (with mounted assault infantry) and defensively (with heavy, slow guns).
My list
Stenberg PDF focuses on the forge world where the battle takes place - I want to portray the many interesting (and dangerous) installations on a Forge that battles can take place around. I'll be looking to follow the core list closely for units and formations, but the list will play much more defensively, since a large fraction of the force will be immobile. It has no particular ambition of being a tournament list, but I hope to make it balanced for friendly play, where scenery matters. By the way, I really like the Curtain Wall; mind if I snitch it?
The
Armoured Division is the Mechanicus on the offensive, possibly on foreign worlds; Less jury-rigged guns and more heavy, reliable, massed armour and elite infantry. Being a lesser threadhead myself, I'll probably buy into it one day, and model up a bunch of tanks too. Though I'll probably wait until a reliable source for Macharius Omegas and CAPSLOCK/ALLCAPS tanks materializes... Those are cool tanks, and they deserve a good home list. This list will play offensively, in an implacable, methodical style.
My
Explorator Mission list focuses on small scientific expeditions encountering hostile aliens (either defending their base or making a drop into hostile territory). It is really mostly intended for scenario games, but I wanted a way of wargaming the more "cinematic" encounters. It is also a way of getting Dropships in, in a setting where they don't skew the focus of the other lists. I do love Dropships, and think that any sci-fi military force should have them, but they have no place in the more defensive PDF lists. This list focuses on aerospace operations, and has very little in the way of heavy support - no Titans, or Majorii, or heavy tanks here, but compensating with cheaper aircraft and an extra spaceship.
E&C: Do feel free to use the Orbiter for the Armoured Division, if you like, they seem similar - thin-skinned, large craft that can ferry vehicles. There's also a Titan Landing Craft, but that's very experimental

The
Forge World Proxima fan list shares the same mission as the core list - it is generalist PDF list, with a slightly more radical feel - nothing Imperial here. I'm not sure I agree with the premise, but it's always useful to get new ideas on the table.
I'm omitting AMTL and Knights, here - while they share a fluff background, they are not "Ad mech variants" like the ones above.
I had a start on an Ordo Reductor list too; I'll scrap that,since it looks like the Armoured Division is filling that space quite adequately.
I hope I've covered the positions adequately; At the very least I hope to have made my personal opinion clear: I think there is space for several different lists depicting the Mechanicum at war, I think it is great that so many people are passionate about developing the Adeptus Mechanicus story, and I'm looking forward to working on the other lists (developing and testing) once the core list is completed and stamped approved.