Internet of Things (IoT) systems are becoming ubiquitous, and their spread has had a significant impact on all aspects of society. Software is a key aspect of IoT systems, from firmware to cloud infrastructures. For this reason, Software Engineering (SE) is crucial to design, develop, deploy, and maintain high-quality IoT systems. Despite the high relevance of these systems, by analysing the literature from a Software Engineering perspective little emerges about their key elements and characteristics, including qualities as perceived by experts working in the IoT field. For this reason, the aim of this work is to understand from the practice, the main characteristics of IoT systems to improve the SE support for their development. We carried out a survey and received 433 practitioners answers from 53 countries across the world. By analysing the collected data, we found that so far: (i) experts working on (industrial) IoT systems only acknowledge in practice some of the main elements and characteristics of IoT systems that can be found in the literature; (ii) most IoT systems require human intervention while advanced learning and self-adaption features are not widely adopted yet; (iii) Smart Industry, Smart City, Smart Building, and Smart Home are by far the most relevant IoT systems application domains; (iv) deployment choices of IoT systems largely favour the Cloud for what concerns the computation (and thus the software); and finally (v) the most relevant quality attributes for IoT systems are reliability, availability, performance, scalability, and security.