I really liked your use case diagram and scenarios, all stated very detailed. But i think your scenarios are a bit longer than needed, that caused some redundancy. For example for your Recommendation scenario, you already wrote the actor is a “logged in user”, but then for your pre-conditions you stated again that the user has to be logged in. These few repetitions make your scenarios a bit confusing, but over all i liked how detail you thought and created your use case diagrams.
Class Diagram
I didn’t understand the need for Relationship class. Besides that I think you can combine RegistrationController and SystemController since they are doing similar jobs and RegistrationController doesn’t actually have many properties.
Activity Diagram
Introduce a Topic: I think a user should be prevented to create a topic without a content and a tag. As your diagram adding tag and content is optional.
Search for a Topic or Post: You didn’t mention about semantic tags or tags here. I think this is important for the project.
Post under a Topic: It would be better if there exists an additional action before entering the text and after visiting the topic.
Sequence Diagram
I think you can include more sequence diagrams for rest of the actions. Besides that the diagram images are very small and almost impossible to read.
Search: I think when the system cannot find an object, it should give an error message to the user, so that the user can understand his/her request is implemented but nothing is found according to his/her search.
Tagging: As i understand from your activity diagram, anyone can add tags to a topic or post anytime. But on your sequence diagram you put a precondition that the user should create a topic or post. I think these two diagrams are inconsistent.
Overall I like the transitions on your sequence diagrams, they make sense and are clear.
Use Case
I really liked your use case diagram and scenarios, all stated very detailed. But i think your scenarios are a bit longer than needed, that caused some redundancy. For example for your Recommendation scenario, you already wrote the actor is a “logged in user”, but then for your pre-conditions you stated again that the user has to be logged in. These few repetitions make your scenarios a bit confusing, but over all i liked how detail you thought and created your use case diagrams.
Class Diagram
I didn’t understand the need for Relationship class. Besides that I think you can combine RegistrationController and SystemController since they are doing similar jobs and RegistrationController doesn’t actually have many properties.
Activity Diagram
Introduce a Topic: I think a user should be prevented to create a topic without a content and a tag. As your diagram adding tag and content is optional.
Search for a Topic or Post: You didn’t mention about semantic tags or tags here. I think this is important for the project.
Post under a Topic: It would be better if there exists an additional action before entering the text and after visiting the topic.
Sequence Diagram
I think you can include more sequence diagrams for rest of the actions. Besides that the diagram images are very small and almost impossible to read.
Search: I think when the system cannot find an object, it should give an error message to the user, so that the user can understand his/her request is implemented but nothing is found according to his/her search.
Tagging: As i understand from your activity diagram, anyone can add tags to a topic or post anytime. But on your sequence diagram you put a precondition that the user should create a topic or post. I think these two diagrams are inconsistent.
Overall I like the transitions on your sequence diagrams, they make sense and are clear.
Good Luck!!