• External schema reference import
• Importing WSDL size limit - 1MB ( utilize middle ware and just consume the middleware wsdl or remove not required wsdl components)
• A single Apex transaction can make a maximum of 100 callouts to an HTTP request or an API call.
• Timeout limitation, can be set for each callout : The minimum is 1 millisecond and the maximum is 120,000 milliseconds.
• SOAP 1.2 is not supported. Use SOAP UI tool to review the bindings
• SOAP UI we can validate WS-I compliance

http://www.salesforce.com/us/developer/docs/apexcode/Content/apex_callouts_wsdl2apex_gen_code.htm

Example to set callout timeout limitation:

docSample.DocSamplePort stub = new docSample.DocSamplePort();
stub.timeout_x = 2000; // timeout in milliseconds

HttpRequest req = new HttpRequest();
req.setTimeout(2000); // timeout in milliseconds