What do you think is the key to a good product review?
Test features in the real world
Be relatable
Be specific about your use case
Know your audience
Be personal
Be empathetic
Point out the overrated features
What’s the mistake you see most often in product reviews?
Always remember that gadget’s don’t have feelings
Don’t fall for the company’s marketing pitch
Don’t be afraid of looking like an idiot
Focus on the features that matter to most people
Don’t get distracted by the numbers
Put yourself in your readers’ shoes
Add your own perspective and opinion
Those are few points that shows how to write professionally for product reviews, I personally likes Honestbeantreview's Review.
Some Lines by Great Bloggers/Brand Reviewers  
It’s simple: A lack of empathy. It’s not easy to put yourself in another person’s shoes in the midst of a product review, but it makes all the difference when it comes to creating something that truly helps your audience. I see product reviews that complain about certain features or hardware changes that likely won’t impact the average user of the product. You have to make sure you’re not introducing confusion and/or alienating your audience by forgetting them. In the end, your audience is who you’re serving in the creation of any product review.
– Mikah Sargent, Producer & Host, TWiT.tv

The biggest mistake I see is being too technical. I want to see someone’s perspective and opinion, and not the specs. Anyone can find the specs on the product website. It’s okay to highlight it but I just find it to be boring if it’s based too much around this.
– Shelby Church, Tech & Lifestyle YouTuber

The biggest mistake people make is getting too bogged down by specs and recounting the promises of a device. If half your review is a spec dump or reads like it could appear on the website for the product you’ve got too far afield. Take a step back and return to asking “what would I or a close friend who’s just as geeky as I am want to know about this product?”
– Alex Cranz, Gizmodo

It happens to all of us, especially when we’re working under tight deadlines, but it’s very easy to fall for a company’s marketing pitch around a product and miss the bigger or more important story with a product. Maybe a company is pushing a particular feature really hard in its advertisements, but that feature either doesn’t matter because nobody will use it or it doesn’t work as well as the company would like you to believe. The best way to combat this is to spend real time with a product and use it in real world situations. Even speaking to people who are considering buying the device or own a prior version of it can help determine whether the new and shiny stuff matters or is just forgettable fluff.
– Dan Seifert, The Verge

How to Write a Great Product Review