This is a list of infamy. These are the things that take away from gaming time, make games take longer, and have people waiting for someone to take their turn. If you find yourself doing things on this list, try to avoid them.
I like players talking during the game. It's meant to be fun, and games are a nice way to catch up with people. What can be a problem is someone talking to you while it's your turn. It's different if you're discussing a trade or something related to the game, but asking me about work or life on my turn is only going to make the game longer. A lot depends on the game. If it's something fun and simple like Can't Stop, talk away. If it's more complicated like Agricola, Cosmic Encounter, or something in that vein, then please, give me time to think when it's my turn.
Sadly, some of my fellow gamers are smokers. This means that they need to go outside and have a smoke every so often. That means that the game grinds to a halt, and then I don't want to be around them with the smell. This may not be a problem for others, but for me, it's an issue.
#5 Game Pieces
Think Poker for this. Players have chips to play with, and do so mindlessly at times. In other games, like Catan, you have a lot of pieces you aren't using yet. This gives you time to build things and play with the pieces. This becomes a problem if you lose track of the game. Also, having spare dice can be problematic, since many players are prone to rolling dice constantly. They're all mindless activities, or just ways to do something, but they can distract others.
One downside of playing games in someone's home is that their kids are around. If the kids aren't gaming, then they're likely going to want something at some point during the gaming time. One little thing isn't a problem, but when they want attention on themselves, it gets frustrating. Games start taking a lot longer, and then eventually someone has to step away to play with the child. The solution to this, turn your kids into gamers.
#3 Significant Others
This can be a significant other playing in the game, or not in the game. For the ones playing in the game, often they will have glances or some other signal when something goes wrong. There also tends to be an unwritten agreement where they don't mess with each other. When only one is playing, the other is often around, bringing snacks, acting all lovey-dovey, etc. It can be distracting for the other players with all of that going on.
Everyone needs to eat sometime, but not all the time. Snacks are a pain because most leave some form of residue on your fingers, so you need to wash your hands before resuming play, which just takes a long time. If everyone takes a dinner break, that's a good thing. People will eat quickly and get back to the table. The problem is the frequent trips to the kitchen to get something to eat.
#1 Cell phones
Specifically smart phones. Sure, they can be helpful, there are all kinds of apps for dice rolling, coin flipping, and some for specific games to track something or help set-up or score at the end. The reasons cell phones are #1 is the people who get on the internet and look up videos, or check e-mail, text everyone they know and so on. What's worse is when they take a call in the middle of a game, and they make it a long call. I've gotten phone calls in the middle of games, and they were important to take, but I try to keep them short. Courtesy is the idea here, and if people have to remind you it's your turn for the 4th time this game, you may want to put the phone away.