If that's all I have to say I generally try to pick out a quote or something that really stood out to me so the author knows why I liked it.

That's exactly what I do, or I'll be honest and say something like "I don't really have anything specific to point out, but I thought it was really well written."

But I totally agree on the liking questions.

At times it's obviously the RL equivalent of pausing and shrugging (so "hm... you know, I'm not sure"), but other times it feels like the equivalent of staring blankly and changing the subject, and you're like... okie dokie then. :P