Shouldn't the title of the thread be "Annoying Multi-player 'Trends/Techniques" rather than "Halo Wars"?
Anyway, I often find annoying techniques in multi-player games; it's one of the reasons I find myself not playing that many of them - and when I do they're usually co-operative games. Some, such as Battlefield : Bad Company 2 and Halo 3, are balanced and/or fun enough to remain relatively rage-free, but games like Modern Warfare 2 are incredibly frustrating due to their poor core design.
A game I have always disliked playing multi-player have been sports games - specifically the Pro Evolution Soccer and FIFA series. Every iteration I've played of both series have included almost game breaking tactics that simply allow the player to score - the worst being the infamous "pass across the goal". For those who don't play football (or soccer) games this simply means when near the goal the opposing payer presses the pass button - and since by default these games have auto-pass on (as in the game guides the ball to your player when you press the pass button) the ball will go past the goalkeeper giving the player an open goal in which to hit the shoot button and score.
It's cheap, effective and incredibly annoying. This tactic has lead to players imply placing their fastest players on the wings of the pitch and proceeding to sprint past defenders before passing the ball into the goal.
These cheap tactics eventually made me force anyone I played against on FIFA09 to change their controller settings from Auto to Manual - so the game could actually have some resemblence of skill and luck rather than shooting from certain positions because they knew the ball would almost always go in from there, or passing across the goal. I didn't get very far though, people seemed to think that passing across the goal was a legitimate tactic rather than a cheap exploit - thus I stopped buying Football games.
