1. The place must be somewhat unique from the rest of the maps. IWD2 has the final battles in Throne room, and later on run to the pool room.
2. it must have some tactical feature, not a flat empty space. IWD2 rooms do have some like that. You can have a few ways to manage your battlefield.
3. It also must have some random factor throwing in, not a THE-ONLY-WAY tactic to win, which is boring. Again, with IWD2, the way randoms infuse that last battle, you dont have a surefire tactic for whole map. One segment or two, fine, but not the whole thing. If the minions chase after you through upper and lower route and you only concentrate on one route, you are screwed. And if they only chase after one route, the resource you deploy on the other route is wasted and you could get swarmed for the time you deploy them back.
4. The adversaries must be hard on their own right, with no obvious weakness. IWD2 two final bosses are tough as fuck. And players might simply want to exploit the game design (if any boss fall down to dangerous HP level, they change to second stage. At 2nd stage, if any are killed, the end game kick in) just so you dont have to drag the battles any longer. Not that you cant defeat them both (even if the game allow), but because you would like to shorten the game somewhat then and there. And depend on your playing tactic, some boss maybe slightly easier than the other. Some game I concentrate on Madea, some on Isae.
In contrast, final battles in Fallout Tactics is not that good. Fighting room to room is not very unique to the whole campaign. Silent Storm got that 3D silo map which simply uniquely to the all three games. Sentinels has you fight three robots at once which is good because you can finally unload all the hoarded HQ explosives on them. Hammer & Sickles has you fighting in a half-expanded, half hilly, half road, unique only in one thing: If you have top of the line sniper, standing en correct spot, you can spot an ambushing sniper on the far side of the map. It's probabbly not in dev's intention but it's not very easy to do so either. Although the second to last, fighting through the tunnels is pretty unique in that game, considering the hidden corridors, the switch etc...