Hyatt Regency is almost like a plain old Sheraton.
Grand Hyatts tend to be ostentatious with lots of faux marbling, columns and trompe l'oeil - suits a certain type of clientele. Service is OK but the size of the hotels makes it difficult to personalize.
Park Hyatts are more boutique hotels with fewer rooms and tend to be more muted and understated. The foreign Park Hyatts such as Tokyo, Paris and Zurich are on par with the best Four Seasons, Mandarin Orientals and Peninsulas. In the U.S., Park Hyatts are hit-or-miss - e.g. that sad conventioneers' hotel known as the Los Angeles Park Hyatt is something to avoid at all costs.