Impressions based on the Japanese version. Xseed and Marvelous already announced the game for western release and in fact it's already playable at their E3 boot.

This is a direct sequel to Fate/Extella, picking up shortly after the final ending of that game. Link is still structured as a musou game where the Servant of your choice must go through a map, composed by several sectors, complete whatever task you need to, and face the boss. Rinse, repeat, complete the game.
I've played a couple of missions in normal difficulty and quickly dropped to easy after the right thumb and the square button went on strike. From what I've seen the difference between easy and normal isn't just how resilient enemies are, but also how many and if Plants will spawn on the map; Plants generate enemies that will then invade other sectors.

While the basic structure hasn't changed, there have been several refinements.
Your character is present on the map and you have to defend him/her/yourself. Normal attackers won't represent much of a problem, but mid-bosses and bosses will be able to damage him/her/you and that will result in a game over (I think, never actually tried). A lot of enemy Servants launch ranged attacks from sectors on the opposite side of the map as well. Unfortunately going through sectors is still a continuous abuse of the dash button and becomes boring very quickly, especially when all sectors are under your control and there are no enemies to fight.
Allied Servants are much more capable than before, and while they'll still require help to defeat opposing Servants they'll now be able to deal with normal attackers with ease. They'll also roam the map trying to help you, and once a boss appears they'll actually join the fray as you engage it. This directly links to one of the new combat features, where stunned bosses will be attacked in unison by all allied Servants currently in the sector.
Combat basics are still the same, two attack buttons plus one meter for a drive attack. R1 acts as a shift to execute four special attacks; Servants gain more powerful special attacks as they level up and you can freely assign them outside missions; one thing is that to trigger these attacks you don't just hold R1 and a face button, but hold R1, wait for the special attack menu to come up (takes a split second), and then push a face button. It's not a particularly big deal, but it does remove some ingenuity from combat.
Noble Phantasms, the most powerful attacks a Servant has, now no lower need three keys to be unlocked but have their own meter. Keys are still present bu they just completely fill the meter, which also fills up with normal combat.
Drive attacks just power up Servants for a short while and the power up can be dismissed early, resulting in a small AoE attack.

With new offensive tools at Servants' disposal killing endless armies is somewhat less redundant, not by much, but at least FEL is no longer a race to fill up the special meter like the first Fate/Extella was. Unfortunately camera is still completely free but you can use L1 to reset it behind your Servant, and lock-on only works on enemies in front of your Servant and there's no way to switch locked-on enemies if not by unlocking and then re-locking, hoping that the system will actually target what you want.
The storyline received a facelift as well. Rather than having a forced series of missions, every stage has two or more missions you can tackle, all connected to a progression chart that leads to different paths. Right now all choices led to the same thing and I was able to go back and clear every mission a stage had, but there are forks in the road leading to exclusive missions.
Missions still have secondary objectives to increase affection level bewteen you and Servants, you can equip skills to get some persistent bonus (but developers ditched icons, so for non-Japanese readers it's kinda difficult to decide what to use), change costumes, and visit the extensive gallery. A new addition is the ability to go around your base, interact with Servants (just a line a dialogue or two), and even go first-person for a more personal view of them.
Graphics have been improved quite a bit, especially locations, and they still move at a fixed 60fps. Unfortunately locations I've seen up till now are the same as in Fate/Extella, just with a new coat of paint, exactly like enemies.
There's a multiplayer component but haven't played it yet.

In short I think Fate/Extella Link will be like Fate/Extella, only with different Servants.