why are archaea in a separate domain from bacteria?

The recently launched three domain classification of living organisms by Carl Woese, says that there are three domains:

• Archaea
• Bacteria
• Eukarya

These domains cover six following kingdoms:

1. Archae bacteria
2. Eu bacteria
3. Protists
4. Funji
5. Plantae
6. Animalia

Before that archaea was considered similar to bacteria but this classification shows that archaea varies from bacteria on the basis of cell wall composition and also from both eukaryotes and bacteria on the basis of membrane composition and rRNA type