As a big fan of FPS, especially stealthy/sniperish kind, I'm highly opinionated about what makes a good sniper game. Actually, ignore that part. I'm highly opinionated about pretty much everything. Yes, that's more like it. So here are my thoughts on what constitutes a good sniper game.
First - the game needs to completely and utterly ignore realism.
You know what real war is like? It's basically walking around in large groups, and not killing anyone for years and years. Most deaths are civilians - by way of intentional killing, not caring much if they live or not, and hunger and diseases that tend to accompany war. Just look at one of the cleanest wars ever - American Civil War. There were 3.1M soldiers, and 625k deaths - that's a fairly respectable 20% death rate (lets skip civilians among them), but only 213k combat deaths - barely 6.7% death rate. Yes - even in a clean war like syphilis is far more dangerous than enemy bullets. In other world, you can expect to kill 0.067 enemies in all four years of that war. Wouldn't that make an amazing game? Play 60 years 24/7 to get one kill. How interesting would that be?
This number is of course nonsense - even most combat deaths are artillery, air bombardment, tanks, machine guns, and other big military hardware. An infantryman's chance of killing anyone is just ridiculously low. It's basically years and years of walking around.
Do you want to know what are the chances of a bullet fired killing someone? The world produces something between 10-14 billion rounds of military small arms ammunition a year. Well, we don't have any war now, but using reasonable (within order of magnitude) assumptions that World War II era's ammo production was more or less the same as today's, and that 20% of military deaths were from small arms fire (as opposed to non-combat and big guns), that gives 1 kill per 17,500 rounds shot. And now imagine a typical game where you're given 100 rounds and sent against a hundred or so of enemies. Ridiculous much?
OK, you might say, but we're talking elite snipers here, not masses of lowly grunts. Then yes, it gets much more interesting indeed. Sniper kill rates are one kill per five round or better. The best sniper in history of the world - Simo Häyhä killed between 500-800 Russians with his sniper rifle, plus over 200 with submachine gun. And he used iron sights, not any fancy hardware.
Still this completely ridiculous number of 700-1000 kills took place over 105 days - that's less that one kill per hour, even assuming he didn't overwork himself and took plenty of rest. And that's so far beyond what a typical sniper can except to get, unless of course it's a video game sniper - those get 5 kills a minute, if it goes well.
Does it mean realism is completely unnecessary, and the games should be using plasma rifles, and rocket pistols instead? Well, that might be cool as well, but reasonably accurate reproduction of battlefield mechanics as an emergent property can also result in reasonably accurate balance of different kinds of weapons, and different strategies, and reproduce richness that would be difficult to program manually.
The second thing we should abandon is Hollywood realism. There are so many ridiculous tropes in movies and video games that are repeated over and over again - people shooting without aiming, or shooting machine guns while holding them in their hands, grenades having ridiculously small casualty range (in most games you're safe if 2-3 meters away, and you need to be pretty much hit in your head to die from a grenade). Fragments from real granades can hit you even more than 200 meters away. And of course in movies wounded people either die instantly, or keep fighting. Nobody is ever wounded in need of hospitalization.
I'm not a fanatic of realism. Real war sucks. If you want some, volunteer to Afghanistan. You don't even need to hurry, that war isn't going anywhere. But often just going the easy way and making it realistic does improve gameplay, so why not do it?
Elements of good sniper games
Pretty much the best sniper game ever is Crysis. Now I'll wait for half of the readers to close this tab. Done? All right. So what makes Crysis such an amazing sniper game?
First, it's a PC game. Sniper games pretty much need mouse and keyboard, no controller gets even close to that. Unfortunately more and more crappy FPS games move to consoles-only, or are console games with crappy PC ports like FarCry 2, what's not much better.
Next, you do have peripheral vision. Humans have plenty of senses they can use to know what's happening in their vicinity. In addition to narrow field of view in front of them, our eyes can see large space around us, though quite poorly. The most practical way to represent peripheral vision in game is with some sort of radar view, just like Crysis does it.
And you can stay camouflaged, while moving at reasonable pace - in so many other games you can spend five minutes quietly crawling through grass, and then being instantly killed by an enemy sniper. In Crysis as long as you're in invisibility mode (which lasts only short time between rechargings), behind cover, hidden in vegetation, or sufficiently far away, you'll be safe.
Enemies behave reasonably sanely, as far as games of this kind go, even though I'd hope for a lot more in the future. They try to get cover, and shoot where they think you might be, and if you're not careful you will die. Crysis did not have to resort to giving them ridiculous aiming powers, or other dirty tricks like that as many other games do.
What's wrong with Crysis
Hey, I like a computer game? That doesn't sound much like me, does it? So let's start talking about what's wrong with Crysis.
First, weapons balance. You're most likely to use the assault rifle for everything. It beats everything else at every distance. It has much higher chance of killing a nearby enemy without aiming than either shotgun or SMG; with single fire mode, decent scope, and Maximum Strength mode it's extremely competitive with sniper rifles on long distances; and it's far better for short to medium distance kills from concealment (where silencer is a must) than anything else. For medium distance burst fire shootouts there's no alternative. And it has far more ammo than sniper rifle and the briefly featured gauss rifle. As a bonus you get a good grenade launcher to instantly get rid of enemy vehicles. There's simply no situation where anything except assault rifle is significantly better than it.
I have some more complaints about weapons. First, ammo is ridiculously sparse. You have to salvage ammo from your kills, and each Korean has just one magazine on him, and no grenades. What of course doesn't stop him from firing infinitely many rounds, and throwing as many grenades as he feels like throwing. It's especially bad with sniper rifles - forcing you to use assault rifles on long range - or move to midrange and use assault rifle from cover, what's actually pretty cool.
Another problem I have is that cover is too strong. Yes, you can blow most kinds of cover like metal sheets, but as long as they stand they protect whoever is behind them. Games like Call of Duty 4 get it right, thin cover offers protection, but not terribly much. I blame Hollywood realism. There's also the usual problem with grenades and RPGs being ridiculously underpowered, even if not as much as in many other games.
There are also some Crysis-specific issues. Invisibility mode, while really cool, can be abused - all energy is lost if you shoot while invisible, forcing you to become visible just like with D&D Invisibility spell. But - if you turn it off, headshot, and turn it on back, you can kill someone in a crowded room without losing any energy! It's an useful combo to learn.
Also rather annoyingly many seemingly cool features are pretty much useless. Tranquilizer darts would be useful for quietly taking down enemies, except they get up in 5 minutes, and set up the alarm - making them pretty much useless. Strength mode for melee attack is quite pointless, as you can as well be invisible, and shoot your target. Speed mode can only be used while high on energy - but if you're high on energy you can get invisible for safety, and if you're low on energy and have to escape, well too bad, speed mode won't work either. Vehicles are pretty much suicidal to use - their weapons have ridiculously low accuracy compared to your small arms, low firepower, and while inside you're a sitting duck waiting to be killed.
That's all still pretty cool for a sniper game.
Games that get it wrong
First, FarCry, no matter how much I liked it when it came out. You have sniper rifle which is good for what it says on the tin, except for its severely limited range. So you had a choice of either assault rifle - which instantly killed the enemy on headshot, but you needed to be close (for headshot), and it was loud enough to alert everyone. And MP5 SMG, which is silenced, but it doesn't even kill on headshot. So in either way, you get detected straight away. But the binoculars/radar mechanism is brilliant (and reused in Crysis).
Call of Duty 4 - there are sniper rifles, but because enemies respawn almost instantly, sniping is completely and utterly useless. You cannot clear the building and then move, or even clear the building, and order your team to move. Just forget about sniping and get the assault rifle back.
Then there are sniper games like Sniper: Art of Victory, which gets everything wrong.
Concealment being hit and miss for no reason, retarded AI, no peripheral vision, amazingly crappy grenades (less than 1 meter casualty range), no penetration of even glass windows, missing point blank shots with SMG, ridiculously low ammo, especially SMG ammo (like 10 bullets), and a very long list of bugs. And these are just some of the gameplay mechanics issues. Boring missions and crappy graphics can be ignored if sniping in fun, but the game is atrocious in every possible way imaginable.
And of course every single console FPS gets sniping wrong by using a controller.
That's it. Now you can go and design a good sniping game. Good luck!