Working at a hedge fund is not like working at an investment bank or a traditional mutual fund. Investment banks and mutual funds are generally large organizations that include support departments that ease the workload of the investment bankers or traders at a mutual fund. For example, there are human resource departments that manage the recruitment of new employees and marketing departments who oversee the marketing of the firms' services to investors and clients. Most hedge funds do not have large human resource or marketing departments -- these responsibilities are instead taken up by the managers of the funds. This also means that employees end up pitching in to help in areas that they normally aren't involved in at I-banks or mutual funds, such as interviewing potential new employees and helping to put together marketing materials.
Because they have a lot at stake with the success of their funds, hedge fund managers are (on the whole) more intense than traditional mutual fund money managers. They are more likely to be involved with the day-to-day running of the firm, which means that the manager will have a higher level of involvement and interaction with most of the staff than at I-banks or mutual funds. Still, as a hedge fund grows, this interaction is inevitably reduced somewhat as the manager hires and delegates some of the operational and trading responsibility to new employees.
On the whole, the culture of a hedge fund is less structured and more likely to wear business casual than an investment bank or mutual fund. Also, the whole firm is usually focused on the success and performance of the fund, which tends to make for a cohesive and collaborative working environment.
Because hedge funds vary widely in size, it is difficult to generalize about the the culture of hedge funds. Many small hedge funds are run like small businesses where the culture of the firm is determined by the owner, or in this case, the hedge fund manager. A fund's strategy also often is a major influence on the culture of the firm. For example, a statistical arbitrage fund is likely to be staffed with Ph.D.s who are less outgoing and enjoy crunching numbers at their terminals. By contrast, a global macro fund is more likely to have a more outgoing atmosphere with the employees watching the markets from a trading floor and more openly sharing ideas. These are two stereotypes of the cultures of different trading strategies and are not applicable to all statistical arbitrage and global macro funds, but the stereotypes still give you an idea about the types of cultural differences you can expect.