In no particular order, probably not even the right set yet...

• Object-oriented Programming
• Component-based Programming (see Scalable Component Abstractions)
• Functional Programming
• Accessibility to Java/C# Programmers - Scala should be an easy “step up” from these languages
• Type Safety
• Performance - performance should be comparable to Java/C#, idiomatic language constructs should not impose significant performance penalties
• Conciseness
• Expressivity
• Readability - Scala should not encourage writing unreadable code
• Extensibility
• Toolability - features should not break the toolability of the language