This is a constant discussion in the veteran community about how tactical a game 40K is. Now remember when you talk about tactics it is purely about how you use the units on the table top. Anything involved in list building is not tactics that is strategics. Tactics is how you move and position your units, who you shoot with what, who assaults what so of course 40K involves some tactics to position units for firing lanes, blocking opponents paths, and positioning assault but how tactically complex is it really. The rules are very complex with all these special rules on units and in area of affects and this level just keeps growing with each new codex release.
Lets look at movement as a good example. What options does a unit have in movement? This is pretty much determined by the unit type during list building. An infantry unit is only going like 6 inches a turn without the random movement of running so for it to cross the board along the short axis it takes 8 turns if it can take a straight path. So both you and your opponent can pretty easily map out where that unit can go in a certain number of turns without assistance from a transport or special rule. Now compare that to a game that might allow the same unit to move 6 inches, 12 inches, or 18 inches in a turn depending on what else it does in the turn. All units in this second game are much more mobile allowing for a much more dynamic battlefield.
Now lets talk about positioning. In 40K your unit positions will determine what it can shoot (be shot by), what it can assault (be assaulted by), and whether it is claiming an objective (or contesting). Almost all of these interactions is pairwise (you and the target) since unit support is all in codex/list specific special rules. Now compare this to a game where if you get a unit on both sides of an opponent then shot with one you shooting is more deadly since they cannot effectively take cover from both units at the same time. Or if a unit near an assault but not actually involved can add some firepower to their side perhaps forcing the opponents heads down while there men move in for the kill. So in this other game you have to be paying attention to the multiple unit interactions not just to know how things will progress next turn but how that will affect this turn.
Now lets consider shooting. There are definately tactics in target selection that will carry over for any game matching weapons to targets they can hit so lets instead focus on how we keep track of damage and unit morale. 40K you lose models which affects a units combat effectiveness(unlike in the Mantic games) and if you lose enough you might have to make a leadership to go between a binary state of full control and combat ready or retreating (ignoring pinned which is a weapon special rule). Now consider a game that has a more graduated morale scale where getting shot at causes a point, watching members of the unit killed causes a point, etc. These points determine whether the unit will act as desired, how combat effective the remaining members are, and at some number they will uneffective. With this type of system deciding who to shot at might becomes more complex since you might be able to do more damage to unit x but 1 point to unit y it will be uneffective.
Now I will talk about 1 other thing. Turn structure. 40K is an all my units go then all your units go type of game. While you can sometimes use special rules in the opponents turn and get to fight in assualt (unlike Mantic games). At the end of each player turn you can look at the whole battle field and start mapping out all the movement and shooting for you units. Now compare this to a one unit at a time game. Now you have to decide an order to use you units and react to each action by your opponent. If he does X then I activate A but if he uses Y then I do B, instead of one big set at the start of the turn you are making the decision 10 times a turn. Add in the ability to try to take 2 unit actions in a row with a penalty for failure and the tactical complexity goes up another level.
Tactical Complexity comes down to how many different things you can choose to do with a unit in a turn and how much you have to consider about what other units are doing also. 40K has essentially move, shoot, assault actions and you can pretty much do them all with certain affects on each other (heavy/rapid fire weapons, having to assault unit shot). You do not really have a ton of choices for each unit which you might find in other games with different classes of movement or shooting ordering like shot then move or hold your fire for the enemy to approach within range.
Now this hypothetical game does seem to have a lot more tactical complexity than 40K. So 40K has some tactics but it is not hard to make a significantly more tactical game. This game seems complicated and must have really long rules but it is not and complete rules including flyers and orbitial barrages, and all army lists are pretty much the same size the the 40K core rulebook without all those codexs full of special rules. What is this game? Epic Armageddon. GW's real battle game in the 40K universe with Titans, Flyers, Space Ships, and everything we love about the universe without the over blown skirmish rules of 40K trying to play a battle game. Take a look the rules are free and armies are cheaper than 40K but given GW pricing in specialist games do not seem like values since everything is priced about twice what it would be for similar amount of material but that 30 dollar titan can be 1/3 rd of a full tournament army.