I hate that Bungie calls their Mathcmaking game mode Optimatch. It's not. Every Xbox game that I've played on Live, if it includes Optimatch, works generally the same way. You specify what type of game you want to play, either generally or specifically. Maybe you want to play CTF. Maybe you want to play a game only on specific boards. Maybe you want to play a Team Slayer game with no more than 10 people in it. In every other XboxLive game that I've played, you choose your game parameters, XboxLive shows you what games are currently forming which match your criteria, and you choose the one which best mattches your preferences. Good luck trying to get into a CTF game on Headlong with Halo2. The first night that the game came out, my friend and I went out to get the game, excited, and were like, OK, let's do some CTF. We coudln't get into one, except maybe every 6th or 7th games. Personally, I'm not interested in being on people's friends list in order to plan time together to find games. I'm not interested in playing 5 game modes that I don't enjoy to get to the one that I do. I'm not interested in playing 3 games that I don't enjoy, building up a party, and then forming a custom game. There are ALREADY custom games out there that people are playing right now, which I'd be interested in jumping in on, but I can't see them, merely because I'm not on the friends list of someone in the game. Why they decided to do Matchmaking instead of Optimatch, I'll never know. It should have been in addition to it. That they still call it optimatch is the worst part.
Little kids who use voice masks. They don't sound older, they just sound annoying.
People on your own team who want a particular weapon, and communicate their desire to you by shooting you as you get close to it, trying to distract you for long enough to grab it themself. As far as I'm concerned, if I jump into a game where one of my teammates starts shooting me, he's trying to interest me in a new game of "shoot your teammates", and I happily oblige him. I'll waste half of that game following him around, and trying to shoot him enough to bring him down to zero health/shields, but not quite enough to kill him.
Team Slayer. I hate it. When I jump into Big Team Battle, it's because I'm looking for CTF games, but don't have that as a menu option. If I wanted to play Team Slayer, I'd choose that option for Team Slayer. I quit out, every time. EVERY time.
Cheaters. I had one game that I played, CTF in Zanzibar, where the other team used a warthog dummying glitch to score a point. It was the only point they got, and we beat them by scoring twice, legitimately, then rubbed it in their faces during the aftergame. Reported the cheaters, then jumped into a new game, and WTF?? The two guys who were warthog glitching were suddenly on my team. They were talking about doing it again, so I grabbed the ghost and just shot the hell out of our own Warthog, blowing it up. And again. And a third time. Eventually, by the 4th time it respawned and they were trying to dummyglitch again, I must have killed them too many times, because I got booted from the game for betraying them. Oh, yea, I also hate that if your teammates are cheating, and you're an honest player, your choices are either to try and tell the other team ("Hey, red team don't shoot, I'm trying to let you know that my t-" BLAM! BLAMBLAMBLAMBLAM! -You were killed by XXXXXXXX-) or to betray them until you get ejected from the match.
I've seen teammates guard a flag capture all the way back to base, then betray their teammate so that they can take the flag and score. That's low.
Me. I'm annoying. If I kill you in game, I'll try to say something appropriately annoying, like "Buh-buyyyyy" or "Eradicator!". If I grab your vehicle off you, I'll scream out "MINE!" like a spoiled kid fighting with their sibling.
