What is "True Line of Sight?"
Basically, you put your eye down at model height and if your model can see it, you can shoot it. Usually, if you can see it to shoot it, it can shoot back as well. Allegedly eliminates all spotting/observation rules and arguments (ha) while being more realistic and cinematic. It assumes models are frozen in place at the moment of measurement. It also assumes all players possess decent, easy to use, LoS-blocking terrain (ha again). It also means a rules designer can ignore "vision" or "observation"rules in favour of a single sentence: "This is true line of sight, guys - if your model can see it, he can shoot it."
Abstract Line of Sight
This often has "size categories" where units are perhaps classed by base size or unit type rather than the actual height or pose of the model. Terrain features like woods are abstracted - you might only see 6" into the wood from the edge (or not at all) regardless if your 'wood' is only three trees on a base (which may have been done to make placing models easier.) It's more accessible than true LoS - heck you can use felt cutouts for forests if you want. Usually, models fully within a terrain piece have a cover bonus, regardless if bits of them are showing or not. The exact pose of the model does not matter, as models tend to be rated by base size and occupy a cylinder or cube of a set size extending up from the base of the model. Abstract rules are becoming increasingly unfashionable. Perhaps this is due to the perceived 'simplicity' 'accuracy' and 'elegance' of TLoS. Perhaps it is also due to the fact having a section on observation/visibility/spotting can remind one unpleasantly of some of the horrifically convoluted spotting charts/rules from games in the 80s/90s (WRG anyone?) Perhaps it is simply because were Games Workshop goes, 90% of rules designers follow.
Using true line of sight, the British soldier is not in cover at all. It kinda makes the 'woods' largely worthless, unless I fill them with so much underbrush it makes placing models impracticable. Using a more abstract system, he could be within the wood if he is on the brown base - which is actually more clear cut than checking if he is partially behind a tree or something.
I'd suggest True Line of Sight can create more issues than it solves. Apparently it makes things simple and solves all arguments - if you can see the target by putting your eye down at the "model's eye view" of your mini, then your mini can see the target. So simple, so elegant! ...in theory. In practice this is less clearcut. I'd say every game I play has at least 1-2 'iffy' TLoS occurances which have to be amicably agreed between players. In abstract systems it's easier to say a force is hidden in a building without checking if the top of someone's head is poking above a window, or deciding what part/fraction of a model counts as in cover. In fact, TLoS isn't strictly TLoS most times - often things are ignored like weapon barrels and flags - sometimes even arms. So why not go further and abstract the whole model? Even hardcore TLoS games like Infinity are now moving to abstract 'base templates' to combat the confusion created by TLoS when models have different poses.
You see, with TLoS, the sculpts of the models themselves make a difference. Kneeling or prone models are harder to see and may have a distinct gameplay advantage, just ftom the way they are sculpted. A cool leaping pose or a model on a jetpack might be easier to spot than a MBT. Even if the jetpck model is walking on foot, with TLoS he's still treated as being 6' off the ground because he's posed up off his base on a wire. True LoS tends to dictate bland, uniform model poses.
True LoS also assumes wargame players don't have imagination. Which, given the slavish adherence to studio paint schemes I see, is probably the case. But it's a bit insulting nonetheless. I don't know about you, but I CAN picture those 3 trees as part of a forest (or at least, a small copse). Heck, I can imagine that painted metal statuettes are actual soldiers and units, which is why I'm playing the game in the first place. In fact, frequently stooping down to see if a model can see between a few trees or through a window breaks immersion somewhat.
TLoS can create a barrier starting the wargame due to terrain requirements. Infinity is notorious for needing a high requirement of LoS-blocking terrain for the game to even be playable. Not all players have the ability to make (or store) tablefulls of terrain.
No cover or hard cover? Under many TLoS rules, the British soldier probably still isn't in cover, as the stone wall is too low (only base height). He's no better off than the German who is out in the middle of nowhere! However it is ludicrous to assume a real soldier wouldn't go prone behind the excellent hard cover and enjoy a massive advantage. This highlights another issue - TLoS rules are often riddled with exceptions to the rule (i.e. half the model obscured counts as in cover, or 1/3rd the model, or 'only torsos - arms and weapons/flags don't count,' etc)
I'd also argue True Line of Sight is LESS realistic. In fact, our game tables are by nature an abstract representation of a real battlefield. Most countryside I know has lots of dips and undulations in the ground - it's not perfectly flat like a bowling green or salt flats. Even 'open ground' has places for concealment. Yet 99% of tables are dead flat with the rare neatly sculpted hill. Battlefields are also usually notable for their noise, dust, smoke and confusion. Yet our battlefields are usually perfectly lit - and we have an unrealistic 'god's eye view' at all times. It's weird we abstract so much, yet insist on "true" "accurate" LoS when it is often detrimental to do so. (And not even very realistic, true or accurate.)
Using TLoS, woods are seldom a barrier (unless we cram them with so much undergrowth that placing models in and around the woods is well-nigh impossible.) What's the point of making terrain that has little effect on the game and is (for many) a nuisance to build and a nuisance to use? A TLoS model placed standing on top of a pile of rubble or a burnt out vehicle is easier to see - where in reality he might be crouching amongst the ruins/wreckage and is in fact in concealment/heavy cover despite his elevated position.
TLoS, I believe, is a reason for our unrealistically short weapon ranges in many wargames. Remember the Bolt Action WW2 rifles that shoot only 24" (50 yards in-scale, and not even the length of Warlord's own Arnhem bridge model!) Short weapon ranges are a (lazy) way of balancing the power of weapons boosted by true LoS.
TLoS also assumes models are static - obligingly "snap frozen" or in stasis at the time of combat. In a era when IGOUGO is on the way out, and fluid reactions, movement and initiative sequences are in, it's not unreasonable to assume that a model is not obligingly standing in that precise location while every enemy empties their magazine at him. Heck - he might move a short distance to seek cover if fired upon, or (gasp!) even go prone without having to be specifically 'ordered' to by his general. This is especially true of "platoon" level games where the basic unit is a fire team or squad that may be occupying a general area rather than a specific location.
There are a few games that do this; where models are assumed to have a modicum of common sense, and are treated as being in some cover as long as they are within a few inches of cover. In fact this could even be made the 'normal' to-hit roll. Only if a unit is stranded way out in the open does it suffer a penalty to be hit (or if it is on a road or similar hard flat surface). Units totally within or behind a terrain feature are fully in cover and get extra defensive modifiers.
True line of sight is an attempt to simplify things, but like removing stats and replacing them with special rules, I'd argue it creates more grey areas that it solves. TLoS is not the Messiah, and it's not a particularly neat boy either.
This is not to say true line of sight does not have its place in some games - but I'd like to question the slavish, near-universal adherence to it (or what I call "Games Workshop sneezes, and everyone else catches a cold").