Having registered in 2004, I know I've seen quite a few changes.
For one thing, the quality of the animation itself no longer matters very much anymore. The days of Adam Philips and RobsH are definitely long gone. The length doesn't count for much either. Your flash could be 30 seconds to 5 minutes long, people almost never judge on the length of content as opposed to what's there.
From what I can tell, the random humor in a Flash triumphs all other aspects. Egoraptor has made a killing manipulating this aspect, as in general, his animations aren't too flashy and don't have the best drawings out there, but their random humor and silly voice acting is what draws people like bears to honey with them. Metal Gear Awesome brought him into the light the way Halo brought Xbox into action. Now, he's got the power to submit just about anything and come out with a score over 4.30 and not even break a sweat. Meanwhile, look at Weebl. The way I feel personally, Get On My Horse simply does not compare to his older work in terms of quality and content, but he's already got a big name and fanbase for himself, so of course he can diminish his quality a bit and not take a serious blow in terms of score. Compare Get On My Horse to Magical Trevor, and you'll see what I'm talking about.
From what I see, the only way to get a major name for yourself in Newgrounds today is to focus purely on very original humor with a unique style that's easily attributable to only you and can not be traced to anyone else on Newgrounds. Also, having your own original characters that are unlike anyone else's is also a must. Your drawings and frame by frame animation must be at least semi-good as well. They don't need to be even close to the stuff Adam Philips did with his Brackenwood series, but they can't look like crap either. Also, working on tributes, such as making yet another Madness flash won't do anything for you, all it does is glorify Krinkles's name yet again. Using sprites, regardless of how good with them you are, doesn't count very much, as I've seen people bash Alvin Earthworm's Super Mario Bros. Z series, even though they're spectacular. Length is not an issue, but people prefer stuff that lasts at least a minute. However, I've seen things under 30 seconds actually score just fine.
Also, the whole "did you find this review helpful?" does NOT mean what it sounds. How those are scored are based on how people agree with the review. You could write a long review criticizing a very popular flash and still give reasons for your low score as well as means to improve, people will still mark it "unhelpful" because they don't agree with it.
This is what I have noticed, but I'm sure other people have their own opinions regarding this kind of stuff.