To answer your questions:
1. oops - looks like an omission. I've reopened https://bugs.eclipse.org/bugs/show_bug.cgi?id=375862 - please feel free to make additional comments on this bug if I've missed anything else :)
2. I'm guessing that you have set the "JBoss jBPM5 Business Process Engine" as the Target Runtime (see the "BPMN2" User Preferences, and Project Properties). This is enabled automatically since it is the only non-default ("None") Target Runtime that is installed with the current version of the editor. Additional Target Runtime plugins are currently in development. Since this runtime engine does not support the concept of a default sequence flow, it is not shown on the property page. Note that you can configure which BPMN2 elements and attributes are visible, in the "BPMN2/Tool Enablement" section of the Project Properties page - check the "Override default tool enablements with these settings:" checkbox and then just select which elements/attributes you would like to enable in the editor. BTW, the "priority" attribute is a jBPMN engine extension which is why there's nothing in the spec about it.
This project is still in the early stages of development and it's very likely that there may be some bugs ;) - thanks for testing. If you find any other problems, please feel free to add them to the eclipse bugzilla here: https://bugs.eclipse.org/bugs/enter_bug.cgi?product=BPMN2Modeler
As I tried so far, It is impossible to create a Text Annotation in a Process diagram.
is that a bug or bpmn is designed in that way?
I have read the BPMN standard documment from OMG. It says that Text Annotation should be available within all BPMN diagrams.
2. and a question regarding the Sequence Flow:
(here i mean the sequence flow from a Gateway and connect to a Task)
in a Process diagram, each Sequence Flow has a priority, but there is no way to make a sequence flow to be default.
In a Collaboration diagram I can create the default sequence flow but there is no priority.
I also found nothing about priority of a sequence flow in the bpmn documment, but it is a nice "feature". I think it's better if the priority of a sequence flow is available for every type of diagram.
P/S i use BPMN document version 2.0
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