The first issue of White Dwarf to have Space Fleet content, issue 139 doesn't just promote or advertise the game, it makes some massive and fundamental changes to the way the game works.
To start with, the whole way ships work was overhauled. The basic Helm Computer, used for plotting movements was replaced by one which allowed faster forward movement and had extra columns for use by super-manoeuvrable ships. Ships now have different speed ratings which affect what options they can choose when plotting movement. The standard Gothic battleship has a speed of two, whereas the Eldar Wraithship from the boxed game now has a speed of 3 and is super-manoeuvrable.
The second massive change to do with the ships was the introduction of data cards. In the basic game, all ships are the same. Same movement, shields, weapons, critical hits etc, which is fine for a basic game, but doesn't really do anything to represent the actual fleets used by the Eldar and Imperium. The new cards allow for all of this to be varied and for the introduction of special rules for various craft. It should be noted that, while this adds a lot to the game, it really doesn't make it any more complex for wargamers to grasp, it just turns it into a 'grown-up's' game rather than one for children aged 8+.
You can see from the above picture how the plastic Wraithships and Gothic Battleships which come in the box are now very different from each other. The Imperial ship is slower and has large broadside batteries. It is also unshielded to the rear, due to the engines. The Eldar ship is faster and more manoeuvrable, but relies on the direction of the solar winds to power it. Its guns are forward facing and it has average all-round shields.
Fortunately, both ships are costed at 100 points, so even if you didn't add anything beyond the ships in the box, you would still have a fair battle to play, just now with very different sides.
As well as these ships, the issue contains data cards for five other Imperial ships. These vary from the massive Emperor capital ship, to the smaller, more nimble Cobra Destroyer and Firestorm cruiser. 'Special' ships are also introduced. The Castellan Shield Ship has very limited offensive capabilities, but projects a strong shield across the area around it, allowing ships nearby to be better defended. Finally, the Ironclad Battleship is a relic from a bygone era, able to take a large amount of damage but having no shield systems at all.
So what about other rule changes? The game is clearly being taken in the direction of larger battles, so rules for squadrons are introduced. These allow groups, usually of 2-3 ships, to be moved with one order. The main restriction is that ships in the same squadron have to remain adjacent to at least one other ship. Oddly, rules for reorganising the position of ships in a squadron is only introduced in the next issue, which means until then, turning a nicely arranged 'V' formation to the right, for example, left the ships arranged more like a 'C', with no way to rearrange them.
With all the new ships, Battlefleet lists were introduced. There were no more Eldar ships until the single one given in issue 140, so it is only really Imperial players who get much choice. Along with picking squadrons from the list, you get 1 commander free for each 250 points in your fleet and can also buy more at 50 points each if you want to have more squadrons
Most of the rest of the article is given to describing other substantial rule changes, such as to the turn structure, ramming rules, fire arcs and weapons. It also include a summary of all the new ships and their special rules. The additional material wraps up with a handy painting guide for all of the currently available ships.
So all in all a very good addition to the game. The biggest downside is that it is quite hard to find an intact copy. The one I was able to get had all of the additional counters and data cards cut out, and the new rules pages loose. Fortunately a copy of the game I got had all the data cards and counters in it, so if you get lucky you might find you end up with everything you need anyway! (Of course, you could always find a PDF copy of the issue on the internet, so or I've been told...)