Simple and fun. You could extend gameplay for a lot longer if you introduced upgrades like different grenades, armor, or more HP. The pause after you throw a grenade is a bit annoying as it defeats the purpose of throwing it (to save time/health). I'd also make headshots stronger to reward players for mouse-aim precision. I'd also add more weapons since you can buy all the weapons fairly quickly.
Not bad, but...
There were a few things that I would change. First off, head shots in any other sniping games I have played have meant instant death to your enemy. This one takes half a magazine of head shots to kill someone. That is a definite problem. The price of weapons is also an issue. The prices should be a bit lower. Also, it is a bit repetitive. A wider variety of enemies will make the game better. It's not bad, but when those issues mentioned above are addressed, it will be a much better game.
The weapons are too expensive.
You should either make them just a little cheaper or pay this goddamn sniper more money. Seriously, if I was payed 10 bucks per body, sitting my ass in the middle of a desert getting bullets from every side, I'd be like 'fuck that shit, I'm going to Canada'.
A wider variety of enemies and backgrounds would make this game a hit: you can make all kinds of bad guys and settings, like hiked Columbian coke mafia in the middle of the rain forest, Peruan 'Shining Path' fanatics, IRA in the hills of Ireland, pissed off ganstas in the dangerous neighborhoods of LA, Somali pirates, Russian bratva, Japanese yakuza, HAMAS, and, for the final boss - Al-Qaeda and the good old Bin Laden, of course!
Although, it's a great game, and I enjoyed playing it a lot.
Good but a little repetitive with some improvement it could be btter.