Tactics games where line of sight is a really big deal, but there's no indication of what you'll be able to see/fire on from a given point.
X-COM, XCOM, UFO:ET, UFO, classic Fallout...
UFO:Extraterrestrials mitigated this by including a wonderful fan modpack which includes a feature, which should be in all LoS tactics games, to simply mouse over a square to highlight enemies that would be shootable from there.
Pretty sure there's a mod for that in XCOM, too, but UFO:ET gets points for hiring the mod maker and including the functionality in the base game.
X-COM it was particularly frustrating because *often* a soldier could see an alien, yet didn't have LoF to shoot. The reasons are complicated as they are silly, and vaguely impressive (they were doing ray-tracing from a specific spot on the soldier where the gun was held). Also the alien could often have LoF on the soldier but not vice versa.
The UFO AfterX series (unrelated to UFO:ET) actually did this best, I think. If a soldier could see a target, but something was blocking the shot, they'd *lean*. Animated and everything. They still needed a clear shot, they were just willing to work for it.
In XCOM, of course, LoS is a complete fuckfest. But at least it's generous, letting you shoot through floors and walls... usually... Of course, that means that hidden thinman will take a reaction shot on the VIP through two walls. Maybe.
Anyway, just venting about LoS in... Fallout Tactics, ironically. I don't understand it enough yet to properly rant about, but the squashed isometric view and slight height differences are making it pretty tough. Not to mention all the cover. I think it's "fair", but it's also hard to predict. And enemies are overwatch-happy, which even triggers on crouching or standing up, so there's a significant cost for each mistake.
Lastly, a different pet peeve... The need to minmax. I love it and hate it.
This is the FIRST MISSION. My main character has an agility of 5 (average) and tagged small guns (very unoptimal for the late game). He's also Good Natured and Gifted, which lower gun skill, but it's still 25%...
He has literally a 0% chance to hit a mission 1 raider at point blank range, with an uzi. That's just dumb.