IEEE 802.15.4, a MAC/PHY protocol for low power and low data rate wireless networks, is emerging as the popular choice for various monitoring and control applications. Depending on the application, the traffic load on an IEEE 802.15.4 network may vary over a wide range. The performance of the protocol, measured in terms of the packet loss probability and the packet latency, depends upon the prevailing traffic load among the nodes competing for channel access, the level of interference from the hidden nodes and the configuration of IEEE 802.15.4 MAC parameters. In this paper, we analyze via simulations the impact of different configurable MAC parameters on the performance of beaconless IEEE 802.15.4 networks under different traffic loads and under different levels of interference from the hidden nodes. Based on this analysis, we suggest the values of IEEE 802.15.4 MAC parameters that results in a good tradeoff between the packet loss probability and the packet latency under different conditions.