Wow! I love that!
What a good idea!
That's a great idea!
Fabulous! Let's do that!
Oh-oh-oh I wanna do that!
I'm not sure. (Meaning: I don't like the idea!)
I don't know about that. (Meaning: I don't like the idea!)
I don't like the idea much.
I think that could be tricky
I think that could be difficult (to work out, to do, to decide...)
That could be complicated.
I wouldn't feel well/good about that...
See also "Giving Feedback on Websites"