A major no-no in a game where you can specialize in stealth and surprise takedowns: Forced boss battles.
In my first run at this game, I dumped everything into sneaking and lockpicking, with just a few spare points in pistol use. My plan was to ghost my way through the game, sneaking around and backstabbing anyone I couldn't avoid. This plan fell apart about three hours in when I met the first boss fight and realized the game expected me to fight a guy in the open, on a bridge, surrounded by henchmen, in broad daylight, with no possibility for a surprise attack. Also, he had an assault rifle. And an OMGillion hit points.
I actually ran out of bullets shooting him.
I started over and created a dull shotgun-wielding lunkhead with more hitpoints than IQ points. It's not a very interesting way to play the game, but it's better than giving myself a concussion from slamming my head against the forced-bossfight wall. And yet even with maxed-out shotgun skills and the best shotgun money could buy, Omen Deng (mister "You're in over your head") took an insane number of shots to the noggin before he finally decided to fall over for good. At the end, the cutscene showed him all bruised. He should have looked like a six-foot pile of buckshot with some bits of meat sticking out.
This could have been the next Deus Ex, but it feels Serious Sam.