Arkham Horror - Tips for making the game more dynamic and fun when you play with more than 5 players?
Arkham Horror is an interesting game in that it lies partway between a traditional board game and a full-on role-playing experience. The atmosphere of the game and the shared imagination of the players is much more important than in games like Agricola or Ticket to Ride, which are dominated by the rules mechanics. It also takes quite a long time to play. 

With more players, it's critical to keep people enthused and involved. This requires active work, by you, if you're the host! Here are the main things I think about.


Set the atmosphere. I like to dim the lights and play horror movie soundtracks. It's obvious, cliched - and fun!
Identify with your characters. We always read out our investigator's back-stories and special rules. And if your players actually talk and act in character, then the game comes alive! If you can provide props or dress up, so much the better. If your players feel like they can relate to and care about their protagonist, they will be much more involved in the game. Staying in character also gives players something to think about when it's not their turn.
Identify with shared goals. If your players feel that they are working together for a common goal, then other player's turns immediately feel more interesting. If the players have the nervous sense that this move is important, then they will certainly pay attention! You want to make them feel that they and their teammates are playing for high stakes.
Know your rules. Nothing kills a game like being bogged down searching for rules clarifications, or having long protracted arguments about ambiguous phrasing. Make sure you have a good grasp of the rules, and if necessary enforce rules decisions to keep the game moving.
Keep the game moving. Some people like to carefully think through every permutation. Some people love calculating attack modifiers. Some people don't care, and want to just CHARGE at the nearest big looking monster. Some people just want to do random cool stuff, infuriating the other players. Striking the right balance between personalities, and gently nudging people along is very necessary. It's similar to chairing a meeting. Do it well, and no one will notice, but everyone will feel the experience "went well".
Read all the cards out yourself. I recommend doing all the reading yourself. It's much faster than passing cards around, and you can add tension and atmosphere by reading dramatically. Your idea of not reading the whole card out is a good one, but does depend on the situation. Remember that some cards in Arkham are optional tests, while others are mandatory. In the optional cases, your player is going to want to know what the risk/reward stakes are. In the mandatory case, it's more fun to drag out the suspense...!
Apply house rules to improve the game. Game designer Richard Launius has provided some excellent house rules which are well worth reading. These can adjust the speed and/or difficulty of the game as needed.
Take breaks. If your players are getting tired or bored, take a break, make some drinks, or agree to resume some other time. Don't turn finishing the game into a death march.
Actively manage the game. This is the critical one. If you're hosting a large number of players, you need to run the game. This is quite a lot of work, but rewarding. Remember that the aim of the game is for everyone to have fun. Even if you are playing an investigator, you still have the extra responsibility for the game experience itself. The ideal game will be tense, nerve-wracking, involve some hardship and sacrifice, but ultimately a positive experience. You want your players to go away feeling they had an adventure, and did well, whether or not they ultimately win. Consider bending the rules where necessary to achieve this.