A simple answer is that hardly there is any comparison as the modern Q&A sites are evolving at a much faster pace, while the traditional forum sites haven't.
- Broadly speaking, here is the summary where modern Q&A sites are better:
- User experience includes a user interface (cleaner and intuitive and high in engagement value)
- Highly interactive and engaging feature set
- Unified Communications (text, voice, video, email- all at one place)
- Semantic/Text Processing, Machine Learning and Artificial Intelligence
- Social Media Integration
- SEO/SEM support/compliance