Let me start with the punchline here. Your opinion does not matter (in design), but your reasoning does. Unless, of course, you're writing the checks, then opine away, please!
The absolute least productive comment you can make during the design process is, "I like (or don't like) it." No one cares. And it doesn't matter anyway. If you're a complete newb to this whole design-gig, you may not realize that "likes" are subjective. Meaning, they are solely based on personal feelings, tastes, perception, background; none of which are universally relevant.
For example, you may like lasagne, because your grammy used to make it from scratch on Christmas morning. I personally loathe lasagne (yeah, go head send the hate mail now). But do either of our opinions make a difference to whether or not lasagne sells to a mass market? NO!
So rather than unhelpfully informing the cosmos that you're (not) a fan, put on your teamwork-hat and come up with reasons why a design is good/bad.
If you have proof that our key demographic is put to sleep by the color pink (interesting read: https://goo.gl/WYvQ4Q), then by all means, present it. If your key demographic is known to have less-than-perfect vision on average (i.e. the elderly), then you probably don't want to do a lot of all-caps titles. And those are the big, easy fish to catch.
There's nothing wrong with questioning the intuitiveness of a buyer journey or the effectiveness of the information hierarchy. Those things are, admittedly, more ambiguous, but they are constructive conversations to have.
I said, in the beginning that it was ok for the client (check-writer) to say they don't like something, but we've all had those clients who gave only that feedback during a review session. What exactly do we do with the fact that you don't like it? Is it too dark, light, dense, sparse, complicated, simple, confusing, uninspiring? The list could go on forever.
So yes, at the end of the day design teams have to pander to the client's subjective likes and dislikes, but it's always easier to do so when those are clearly spelled out. "I don't like so much plastic on the toaster. It makes it look cheap. I want it to look luxurious, like Mario Batali uses it in his kitchen." That I can work with.
And if your client doesn't willingly serve up those extra details, you have to find a way to diplomatically mine for them. One of the safest ways to do this, is to return to the original design criteria; usually spelled out in a creative brief. "Ok, so we know we needed this design to [insert specified need]. Do you feel like we missed the mark here?" Giving the client something precise to hone in on may focus a scattered or insecure thought-process. And you might start to make some traction.
NON-DESIGN TEAM MEMBERS
If you're on a team, but not a design professional, please don't think that your comments have less validity. It's not unusual for designers to get wrapped around the stylus. We get tunnel vision too. Which is why the best teams have many disciplines represented.
That being said, you are still held to the same rules of the game. You may not have the industry jargon up your sleeve, but don't ante up with an "I (don't) like it."
Let's assume for a moment that you're a writer or content manager. If that's the case then lean in to it. "I don't think this is enough supporting information. We have very intelligent, highly-demanding clients. And showing them something with so little substance will probably insult them." Beautiful. Love it! I don't care that you just told me my design looks like a rat-turd. I can work with that very constructive criticism... Assuming you're willing to provide the extra verbiage, that is.
Now, you're a production person. You're point of view will be different obviously, depending on your deliverable. Is your team creating a website? A coffee table book? A new surgical tool? Whatever it is, your input is desperately needed. Here's a tip though. Whoever you are–I can pretty well assure you–that you are known as the "No-person" on your team. "No, that's not feasible." "No, we could never get that done in time." "No, I don't have enough resources for that." All totally valid and necessary points, but what I'm getting at here is, you especially need to have reasoning on your side. Otherwise your teammates are just going to assume that your default answer is "No," and you're not really trying to help brainstorm or contribute to the end goal.
One of the least helpful things I've had a developer say to me was, "No, we can't do that." Period. End of sentence. What the heck? First of all, tell me why we can't do it, so I can learn and not make the same mistake in the future. Second, you understand the client's request just as well as I do, so if we can't do it my way, suggest an alternative.
On the flip side, one of the most helpful things I've had developers do is eagerly engage in brainstorming. "Our code base won't allow for that custom configuration, but, with this new plug-in I tested the other day, we could allow the users to enter a response and get a poll result instantly." Awesome! That wasn't my original plan, but I can work with that. And since it's a plug-in, the build time will be shorter. Win-win.
PROJECT MANAGERS/TEAM LEADS
You are considered the closest to the client. You should have the best relationship with them and be fully committed to championing for them in their absence. Just as you're fully committed to championing for us, your team, when we're not with the client. Tough job, but you're good at it.
Because you wear many hats, you are probably the best suited to look at the design with fresh eyes. Consider what the client wants and has been happy with in the past. And then consider what is new and innovative enough to merit pushing for anyway. But most of all, participate, in meaningful ways. "I know the client has always preferred we use their local printer, but if this other company can do what you're describing, it's worth pitching. I think the client's booth will stand out against the competition at the expo with this new addition."
The simple ask, here, is this: throw "I like" and "I don't like" out the window when you are working on a design. Try for a moment to forget what Grammy did on Christmas Day or any of the other subjective opinions that make you, uniquely you. Come to the table armed with your education, industry experience, logic, rationale, research, a constructively-critical mind, an enthusiasm for brainstorming and a respect for all of the perspectives/disciplines sitting around you. Because as much as the world thinks of "design" as the task of making something "pretty." That's not it at all. It's about making a thing great, usable, effective, intuitive, sellable...