1. Bosses (or mid/bosses) that don't have a health meter. so you never know or have to guess whether you're doing any damage to it.
2. Bosses that don't acknowledge your hits. games give you all the combo strings in the world to use but the bigger enemies just power up their super unblockable moves and KaPow. regardless of how much hacking and/or slashing you may be doing.
3. games that give over-powered enemies free hits after knocking you down. It's bad enough there are MANY more of them than there are of you, this is just unnecessary humiliation.
4. Kill one guy,end the war. you may have an entire strike team of soldiers with you, but all enemies are aiming (and firing) at YOU. even your flanking manuevers are ineffective while your co-fighters/wingmen/minions are shooting at nothing and not getting shot at at all.
5. Find all the hidden things. Coins/Flags/Statues/Intel/whatever. i know these things exist just so the programmers will force you to see all the locations and graphics that they took the time to put into the game but c'mon, some of these are so deviously hidden they require you to go on youtube to find out how some other smarta$$ did it.
6. Games that rate/rank you. If i have to camp in one spot and kill every guy that comes around a corner i will do what i have to do to overcome the incredible odds put against me. Then at the end of a level, the game gives a big middle finger to you and gives you a D rating for the tactics you used.
feel free to add to this list any things that annoy you about modern gaming.
(be advised,this list must NOT contain anything about online gaming. that is a WHOLE OTHER catagory.)