In terms of what you should have in a review:
While you're reading a story (especially a longer one) you should write down/remember specific things that you enjoy, or places with issues. Honestly, a good review just needs to talk about a few details of the story or writing, and that's enough. Feel free to be scathing in your critiques, although be realistic as to whether or not this story should be given scathing critique. Generally, a featured story is featured for a reason: if your comment is just bashing the story without talking about any of the good things, then are you really reviewing in a holistic and unbiased manner? I also always look at word count and judge how much effort was put into a game. Usually, "baby's first story" which is a thousand words about zombies or dragons or some bullshit, gets no comment or a brief, angry attack. However, a masterfully crafted game (e.g. Mystic_Warrior, ninjapikta, Endmaster), gets a comment with as much detail as I can muster. If those authors can put the effort in to create a 100k word storygame, I can put in the effort to create a thoughtful, 500-1000 word review, and they really deserve nothing less. I'll also put care into reviews for lesser storygames that have had some thought or effort put in, or those that show a promising future from the creator (if the creator shows promise, there's no need to savagely berate them, although you should still include criticism so that they don't think they're the hottest shit since the one I took last night).
In terms of comment style:
Some people have exceptionally witty comments, but this isn't necessary at all (although if you're able to write these, then bravo). My personal, tried and tested style is that I leave a rough summary first, then a more in-depth spoiler section afterwards. The general section is tailored more towards a random passerby wondering if they should read this storygame, and the spoilered section is directed towards the author (or others who have finished the story and are looking to discuss [lol]). In general, it should be easy to avoid writing fluff reviews, and if you find yourself repeating what you've already said, then you should just give the review a once-over, make sure you've gotten down everything, and post it. There's no point in writing an extra long review for the sake of being extra long, it's just a waste of time to you and anyone who reads it.
In closing:
Go forth, read, and I better see some solid reviews out of you with all the advice you've been given.