Let's take an EAR developped solely in Websphere Application Studio Developper (WSAD) with no external plugin.

Deployment on JBoss 3: OK

Deployment on Websphere AS 5 (WAS):

[04/04/03 11:46:48:297 CEST] 666d11fd WebContainer  A SRVE0169I: Chargement du module Web : MYAPP.
[04/04/03 11:46:48:537 CEST] 666d11fd ComponentName > populateJavaNameSpace for web app
[04/04/03 11:46:48:537 CEST] 666d11fd ComponentName > populateJavaNameSpace
                                 javax.naming.InitialContext@5f1611e9
[04/04/03 11:46:48:598 CEST] 666d11fd WebApp        E SRVE0015E: Echec de l'initialisation de l'application Web MYAPP
[04/04/03 11:46:48:698 CEST] 666d11fd TraceNLS      u No message text associated with key 
Error.Loading.Session.Context.and.Web.Application.{0} in bundle com.ibm.ejs.resources.seriousMessages
[04/04/03 11:46:48:698 CEST] 666d11fd WebGroup      E Error.Loading.Session.Context.and.Web Application
 java.lang.NullPointerException
	at com.ibm.ws.util.ComponentNameSpaceHelper.populateJavaNameSpace(ComponentNameSpaceHelper.java:587)
	at com.ibm.ws.util.ComponentNameSpaceHelper.populateJavaNameSpace(ComponentNameSpaceHelper.java:276)
	at com.ibm.ws.webcontainer.webapp.WebApp.populateJavaNameSpace(WebApp.java:1154)
	at com.ibm.ws.webcontainer.webapp.WebApp.init(WebApp.java:243)
	at com.ibm.ws.webcontainer.srt.WebGroup.loadWebApp(WebGroup.java:317)
	at com.ibm.ws.webcontainer.srt.WebGroup.init(WebGroup.java:194)
	at com.ibm.ws.webcontainer.WebContainer.addWebApplication(WebContainer.java:950)
	at com.ibm.ws.runtime.component.WebContainerImpl.install(WebContainerImpl.java:133)
	at com.ibm.ws.runtime.component.WebContainerImpl.start(WebContainerImpl.java:360)
	at com.ibm.ws.runtime.component.ApplicationMgrImpl.start(ApplicationMgrImpl.java:401)
	at com.ibm.ws.runtime.component.DeployedApplicationImpl.fireDeployedObjectStart(DeployedApplicationImpl.java:743)
	at com.ibm.ws.runtime.component.DeployedModuleImpl.start(DeployedModuleImpl.java:337)
	at com.ibm.ws.runtime.component.DeployedApplicationImpl.start(DeployedApplicationImpl.java:531)
	at com.ibm.ws.runtime.component.ApplicationMgrImpl.startApplication(ApplicationMgrImpl.java:254)
	at com.ibm.ws.runtime.component.ApplicationMgrImpl.start(ApplicationMgrImpl.java:232)
	at com.ibm.ws.runtime.component.ContainerImpl.startComponents(ContainerImpl.java:343)
	at com.ibm.ws.runtime.component.ContainerImpl.start(ContainerImpl.java:234)
	at com.ibm.ws.runtime.component.ApplicationServerImpl.start(ApplicationServerImpl.java:117)
	at com.ibm.ws.runtime.component.ContainerImpl.startComponents(ContainerImpl.java:343)
	at com.ibm.ws.runtime.component.ContainerImpl.start(ContainerImpl.java:234)
	at com.ibm.ws.runtime.component.ServerImpl.start(ServerImpl.java:182)
	at com.ibm.ws.runtime.WsServer.start(WsServer.java:135)
	at com.ibm.ws.runtime.WsServer.main(WsServer.java:232)
	at java.lang.reflect.Method.invoke(Native Method)
	at com.ibm.ws.bootstrap.WSLauncher.main(WSLauncher.java:94)
	at com.ibm.etools.websphere.tools.runner.api.ServerRunnerV5$1.run(ServerRunnerV5.java:105)

[04/04/03 11:46:48:778 CEST] 666d11fd DeployedAppli W WSVR0206E: Le module MYAPP.war de l'application
MYAPPEAR.ear/deployments/MYAPPEAR n'est pas parvenu à démarrer.
[04/04/03 11:46:48:788 CEST] 666d11fd ApplicationMg W WSVR0101W: Erreur lors du démarrage, MYAPPEAR
[04/04/03 11:46:48:798 CEST] 666d11fd ApplicationMg A WSVR0217I: Arrêt de l'application MYAPPEAR
[04/04/03 11:46:48:928 CEST] 666d11fd ApplicationMg A WSVR0220I: L'application est arrêtée : MYAPPEAR

And what would happend if I try this on an IBM OS? WAS won't even start?