HI,

I've taken a screen shot from my game and added a bunch of mockup elements. The white text gives a brief description of each element.

Note: the laser turret and the tank are the wrong colour, they will be coloured in the colour of their faction - red for the turret and blue for the tank. (You drive the tank).

Without notes:
clixxor

With notes:
clixxor2

I want to get some opinions on the following things:

1) The crosshair. I'm not sure it works, and need ideas for what might be better.
2) Are the overlays too opaque? Should I fade them out a bit more?
3) Are the reload / heat indicators a good idea? The big idea is that slow reloading weapons get a number of red dots (5 being fully reloaded) and that faster firing weapons have a heat bar (green bar shrinks and changes to red) or a recharge bar(bar remains green, and shrinks in size)
4) Is green strikethrough enough to indicate that an objective has been reached?

I don't know why, but I prefer vertical health bars. I think they are easier to see at a glance, and may stem from viewing glasses of liquids from an early age.

I would also consider putting more info in a nice transparent HUD to either side of your center view, and moving the health bars there. However this is something that needs to be played with carefully to avoid obscuring target acquisition.

1) The crosshair. I'm not sure it works, and need ideas for what might be better.

I'd suggest make the red bounding rectangle to be smaller than the size of the target (maybe fixed size? so objects that are really close to you won't have a giant rectangle over them), and put all the information outside of the rectangle

2) Are the overlays too opaque? Should I fade them out a bit more?
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Can't say much right now as the game is nowhere done. Are you going to texturize your polygons or leave them the way it is now? This depends on the background color. Fade them out more if the overlays seem too "striking" to you.

3) Are the reload / heat indicators a good idea? The big idea is that slow reloading weapons get a number of red dots (5 being fully reloaded) and that faster firing weapons have a heat bar (green bar shrinks and changes to red) or a recharge bar(bar remains green, and shrinks in size)
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Yes, but I prefer them to be part of the cockpit UI, rather than floating arbitrarily on the screen.

4) Is green strikethrough enough to indicate that an objective has been reached?
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Yes, but it doesn't match the LCD font look. You are using that calculator font, but when was the last time you see a calculator able to render a diagonal strike-through like that? Either change the font to something more modern, or make everything as though it was rendered through a 7-segment LCD display.

1) The crosshair. I'm not sure it works, and need ideas for what might be better.
2) Are the overlays too opaque? Should I fade them out a bit more?
3) Are the reload / heat indicators a good idea? The big idea is that slow reloading weapons get a number of red dots (5 being fully reloaded) and that faster firing weapons have a heat bar (green bar shrinks and changes to red) or a recharge bar(bar remains green, and shrinks in size)
4) Is green strikethrough enough to indicate that an objective has been reached?

1/3) At first, the placement of the reload/heat indicators make sense, this is the logical place to have them as this is where you are looking, and it saves having to scan the screen to find them when in a battle. However, I cannot think of a single FPS that puts them here, and indeed even military combat HUDs do not place them here, but instead place them to the side of the screen. i would imagine others have done a lot of research on this (particularly the military), and must therefore have found a good reason to not put them here... perhaps due to them providing too much clutter and making the target harder to see and identify. I would stick with conventional wisdom here and locate them elsewhere.

Following on from that, the actual crosshare seems a little too large. Typically the crosshare should give some indication as to the spread of the weapon - does your crosshare do so? It suggests that the guns spread decreases with distance, and the projectile drops with distance. Is this the case?

4) While this does work, I think a tick next to each item would work far better, and be much more instantly recogniseable. Is it necessary to display completed objectives, or should these clear once complete to free up some screen real estate? Also, red to me suggests failure rather than an objective waiting to be completed. I would suggest going with a neutral white/grey for remaining objectives, and red for an objective that has been failed or can no longer be completed.

4) Is green strikethrough enough to indicate that an objective has been reached?

To my poor-ass eyes (diabetic retinopathy) the red text is a little too dim to read without staring at it for a moment, fully ignoring the rest of the screen. Which may be detrimental to player survival ;) My ease in seeing stuff is roughly similar to the contrast it would have after being converted to greyscale, and reds always end up darker after such a conversion.

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4) Is green strikethrough enough to indicate that an objective has been reached?

To my poor-ass eyes (diabetic retinopathy) the red text is a little too dim to read without staring at it for a moment, fully ignoring the rest of the screen. Which may be detrimental to player survival ;) My ease in seeing stuff is roughly similar to the contrast it would have after being converted to greyscale, and reds always end up darker after such a conversion.
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i agree the red is a bit hard to see, and in a combat situation it could even be missed.
although it might be just because its against a black background, it would proberbly stand out if the background was white or something brighter.

I'm with Luckless on the health and shield bars. I'd rather them be on the sides than on the bottom. Though I'm happily used to them on the top of the screen anyway.

1) The crosshair. I'm not sure it works, and need ideas for what might be better.
I think a circle would work better than a rectangle and typical "cross" crosshair.

Like:
http://thestranded.files.wordpress.com/2011/04/brink-hud.jpg
or
http://purenintendo.com/wp-content/uploads/2009/08/metroid-prime-3-7.jpg

Though the metroid prime look is a bit too faded for my tastes...

HUD elements should be sleek... your current red rectangle look comes off more like MSpaint.

2) Are the overlays too opaque? Should I fade them out a bit more?
Opacity looks fine. You need to ramp up the brightness though. These are digital display elements here... if it were LCD they would be bright and clear to see. I'm going to say that even on a bright background these would be hard to read... so definitely ramp up the "glow" or "brightness" on these. Also giving players the option to tweak these settings would save you a lot of grief also... Just let players adjust opacity and brightness to their own personal tastes and your problem is mostly solved.

3) Are the reload / heat indicators a good idea? The big idea is that slow reloading weapons get a number of red dots (5 being fully reloaded) and that faster firing weapons have a heat bar (green bar shrinks and changes to red) or a recharge bar(bar remains green, and shrinks in size)
Would probably need to see it in action, really.

4) Is green strikethrough enough to indicate that an objective has been reached?
Yes, if it were brighter. I'm not sure about doing it diagonally though... It's leaving a lot of letters practically un "struck" so it is distracting. Just have it strike clean through the middle.

Some good feedback here, thanks all for your responses.

I'm going to implement the following things, based on suggestions I've been given:

1) The mission text will be on against blackish (multiplicative blended) rectangle to create greater contrast
2) I'll forget about the heat / charge bars on the crosshair, instead putting them along the bottom of the screen
3) Due to other reasons, I've changed the crosshair in the third person view to a laser pointer which is activated by a light press of the main trigger. (Crosshairs don't work too well in a third person view when the camera is offset from the barrel)
4) I'm going to change the red border around the currently highlighted enemy to corners only.

The bars around the crosshair are something I first saw in half life 2, which put health and ammo bars to the left and right of the crosshair, respectively.