Sometimes when I visit sites for something specific thing I seen "Is it helpful?" link but I never hit it as yes or no. When people searching for something specific and if they will found in Stack Exchange then they'd prefer it , Even when I found something helpful with SE sometimes I'm glad as I'd like to appreciate it ; so I'm thinking if there were feature like "Is it helpful?". But also I'm thinking as for just nice appreciable question and answer ; otherwise they'll mess it with unanswered questions and incomplete answers. Only with positive perspect . "Appreciate Answer". Sounds so much but wish I could see it.
When you're unregistered or without enough reputation to vote, clicking a vote button still does something. The feedback really is stored. Moderators and >10K users have access to a "post feedback" information page. There, they can see - for example - questions that are voted up much higher by real votes than by anonymous/low-rep feedback.
If you really liked a question, you can star it as a favorite regardless of how much reputation you have. Admittedly, this doesn't really affect anything other than letting you easily find the question again and possibly giving the question owner a badge.
I see you're getting close to 200 reputation on one site. Once you get there, you'll receive the association bonus (+100 on every Stack Exchange site you join), which will enable you to, among other things, comment and vote up.