Too many clicks to get data. E.g., my return date should automatically be later than my from date. I shouldn't have to click on the >> 10 times to get to Dec.
Open up too many browser / tabs. TripAdvisor sucks at this. Every hotel click opens a new tab.
Too fancy, fonts too large, too many scrolls up and down...etc. In other words, give me something straight forward and simple. Like Google Flights.