The main challenge for some people is adjusting to the climate. Singapore has a tropical climate, which sounds very nice (and sometimes is great!), but it is almost on the Equator (just 85 miles away!). This means it is very hot and humid, and there are no seasons (no spring, fall, or winter--just summer!).
Singaporeans have adapted to the environment by making great use of fans and air-conditioning, which are very common (without which the heat would be nearly unbearable). Additionally, Singapore recently adopted the Singapore Green Plan in 2012, which seeks to promote conversationist efforts and using green technology. This is why Singapore has such clean air and water as well as great waste management.