Ajax is primarily a client/server technology. Client nodes in the web do not typically run web servers that can respond to HTTP GET and POST requests. Since clients cannot directly communicate with each other, any distributed application developed usingAjax would require a central web server system to negotiate requests between clients. Client A could generate events/data for and respond to events/data from Client B however both would need to connect to a common web server to facilitate the communication.
Another limitation of Ajax is slow or unreliable network connections. Traditional web applications that require a full round-trip to the server may behave slowly for end users but they would be predictable. End users would generally intuitively know they have performed an action, such as submitting a form or clicking a link, and must wait for a server to respond. With Ajax, users  have less intuition about which type of events within a page may result in a need to wait for a server response. Instead of enhancing the end user experience, Ajax over slow or unreliable network connections may reduce usability if not carefully designed to accommodate both fast and slow network characteristics.