Using my 1750 Marine list I've deployed everything right on the 12" deployment line and spread my forces out as much as possible. This gives the perceived advantage of allowing me to fight anywhere on the battlefield as I have units spread all across the board and can easily bring firepower to bear anywhere. However, the opponent can easily deploy on one side of the board and effectively deny some of the Ultramarine firepower as they actively refuse flank them. Here is an Eldar list doing this:
Whilst only the far left Dreadnought and Land Speeders are really out of the fight for the first turn (and the Dreadnought can move into range) the Space Marine army is spread out and is going to have a much harder time bringing maximum firepower to bear on the Eldar at any given time. On the other hand, the Eldar army due to deployment and their speed are quite capable of moving as a single 'unit' and flanking the Space Marines (i.e. moving up the right flank and shooting at the tanks, particularly side armor). Once the Marines are flanked the far units (left flank) will have a much harder time getting clear shots on the opponent and the Eldar army can work their way across the field by moving right to left along the Space Marine battleline. Since the whole Eldar army is moving as one, the Space Marine army is going to have a hard time stopping it in single parts.
Essentially what deploying in a line can often lead to is a movement disadvantage. Deployment is basically a free move for any unit, anywhere within a specified area. If you spread out over the whole area you may have 'gained' a bunch of free movement but you have to use your normal movement to try and get back into better positions. This isn't always possible (24" move armies can often do this aka: rapid redeployment).
This mobility issue is compounded by the Space Marine's minimal effort to take advantage of terrain. The Preds and Razorbacks as well as the Speeders + Dread on the wing (depending on the height of the ruins) are going to have a hard time generating cover from directly across themselves. In this example, the whole right flank isn't generating cover and once the Eldar push up the right flank with their superior mobility and move right to left along the Space Marine line, cover is going to be hard to generate as well. This is what you need to take into account during deployment.
Let's look at some of the advantages of line deployments then as there are some. If your army is fast, i.e. like the Eldar army in the above example, deploying in a line allows you to meet any threat anywhere through rapid redeployment. You lose most of your firepower on that turn but your army is able to move rapidly and strike where it is needed. For example if Eldar got the first turn and deployed with the Space Marine player reacting and deploying like this...
The Space Marine player has attempted to maximise their firepower advantage at range by deploying on the opposite flank of the Eldar but due to the Eldar's speed, they can sacrifice some shooting to re-engage the Space Marine castle by a 'rapid redeployment.' This brings the Eldar army to bear for T2 whilst allowing some shooting in T1. Whilst the Marine player is now engaged, they have gained a precious turn of shooting and movement before the full Eldar force is able to engage. Here we can see the redeployment of the Eldar army and their speed.
When Turn 2 beings, the majority of the Eldar forces will already be in range and can therefore use their movement to try and gain advantageous shots rather than simply getting into range. If the Space Marine army have been in the same position they'd of had a much harder time moving to engage a list which has better shooting than them due to their slower speed.
NB. I know these are basic deployments and we hopefully wouldn't see such regularly but I'm doing this on the fly!
Another advantage of using the line deployment is ensuring maximum distance achieved. This is very important for armies which want to move across the board against particularly shooty armies (i.e. any army against Tau). This does not mean deploy out in a big long line like in the first picture but rather ensure the elements of your army which are likely to be the most effective if they can move across the board (i.e. transports with guys inside them) can move the maximum distance if they are not shot up. This means put them on the 12" line. Obviously try to gain them cover from terrain if possible but if you have the units you want to move across the board providing cover for each other and the front line gets wrecked or immobilised, your opponent has gained a turn as you go around the immobilised vehicles or damage yourself on the wrecks. The rest of your army can of course deploy as they wish but when units need to get across the board, line deploying those units in specific circumstances can be a viable option.
Conclusions
All in all deploying the whole army in a line across the board is going to leave you at a tactical disadvantage to your opponent. They are more able to target a specific point in your army and bring their force to bear on that point whilst your army is going to scramble to react. Some armies can line deploy to an extent due to their favorable manoeuvrability whilst other portions of armies should seriously consider deploying in a line to ensure no matter what their opponent's shooting does, some elements can still move forward and threaten their opponent reliably.
In the end, simply lining your army up on the deployment edge with little thought for future movement or shooting and not taking the opponent's army or terrain into account is going to leave you at a significant disadvantage. The line deployment is probably the most common executions of this lack of thought.
We all remember what happened when Taak did it versus me after all :p
