Nice, I’m glad to see you’ve made the next steps!
I worry that you may not have chosen the least controversial place to start. 
So don’t get discouraged if that particular “small” change gets bogged down. Lots of people will have strong opinions about the first words that a site visitor will read.
I’m still hopeful, of course. But if that particular change stalls out, there are lots of other things that could be improved. Getting other changes through would help improve the level of trust the committee has in your work, which would make it easier to get consensus in the future.
I also have strong opinions. 
My own suggestion for a better tag line would be “A radically different paradigm for fun and maintainable software”. It hints at the fact that Haskell is truly different, and hits the two main benefits that appeal to me about the language. I’d want to follow up with facts about its “seriousness” to offset the danger of being mistaken for a toy, but I’d definitely want to start with a pure expression (lol) of why Haskell is valuable.
For inspiration, I think https://sass-lang.com/ is especially good. It quickly paints a picture of what Sass provides, and follows up cleanly with features of the language and community. Unsurprisingly, it looks good, too.
For comparison, here’s a bunch of language website tag lines:
- Rust: “A language empowering everyone to build reliable and efficient software.”
- Python : “Python is a programming language that lets you work quickly and integrate systems more effectively.”
- Ruby : “A dynamic, open source programming language with a focus on simplicity and productivity.”
- Scala: “A programming language that scales with you: from small scripts to large multiplatform applications.”
- Go: “Build simple, secure, scalable systems with Go”
- PHP: “A popular general-purpose scripting language that is especially suited to web development.”
- Fortran: “High-performance parallel programming language”
- C#: “The modern, innovative, open-source programming language for building all your apps.”
- Swift: “Swift is a general-purpose programming language that’s approachable for newcomers and powerful for experts.”
- Java: “Oracle Java is the [number one] programming language and development platform. It reduces costs, shortens development timeframes, drives innovation, and improves application services. Java continues to be the development platform of choice for enterprises and developers.”
- TypeScript: “TypeScript is JavaScript with syntax for types. TypeScript is a strongly typed programming language that builds on JavaScript, giving you better tooling at any scale.”
- Matlab: “Math. Graphics. Programming. MATLAB is a programming and numeric computing platform used by millions of engineers and scientists to analyze data, develop algorithms, and create models.”
- Sass: “CSS with superpowers. Sass is the most mature, stable, and powerful professional grade CSS extension language in the world.”
- Octave: “Scientific Programming Language”
I find them to be a mix of outcomes, features of the language, and benefits of using the language.