1) Waste and waist are different words, as are bare and bear, through and thorough, and many other homophones. These are important word in smut; get them correct. Dictionaries are your friends. Please, please use a beta reader/editor, or at the very least let your writing "rest" for 24 hours before you proof-read it yourself.
2) Do not go from Harlequin romance euphemisms like "hardness" and "manhood" and "released his emissions" to "dick" and "cum" within two paragraphs. It's jarring. Pick a tone and stay with it, unless you have a POV change or obvious mood change.
3) About the sex...
a) Frosting, wine (or anything alcohol-based such as beer or liquor), honey, cookie dough, maraschino cherry juice, and dew are not lube. If you've ever had anything in either of your lower orifices, you would know this. Unless your fic is a worst-lube-ever piece, don't use bad lube just for the sake of creativity. Far –far- better to just gloss over it with a brief phrase saying that one prepared the other before penetration, than to use something so plainly absurd.
b) If at all possible, stick something up your own ass before you start writing about anal sex. A soapy finger in the shower is a good start. If you haven't ever felt something first-hand, and have no idea what you're writing about, your readers can tell. Honestly. If you live in a place where you cannot buy sex toys, let me know and I will send you some. Even better, try the tried and true vegetable substitutes. Think of it as an educational experience that will help your writing, whether you get off on it or not.
4) Plot is good. Plot is nice. Plot can make hot sex fantastic. However, BAD plot that is worse than the scripts in porno movies is WORSE than no plot at all. Do not introduce 15 new characters into your story when their only purpose is to delay the two main characters getting it on. After the 30th chapter, I lose interest, no matter how good you are at writing the steamy scenes of lustful longing. If you take too long getting there, no one's going to stick around to see it.
5) If you co-write a lot of fics and get rave reviews on them, but not on your solo pieces... try to figure out how the pieces are different, content or style, and see if you can improve your own skills so people think of you as a good writer on your own, not just as part of a team.
6) On plagiarism:
a) If someone says you're stealing their ideas... at least have the honesty to admit it and go back and add credits to them in the fics you've written. Apologize. Try to not do it again. No idea is ever original, but do try to think about if you saw something somewhere before you write it into your own story. Ask, attribute, and thank the original inspiration.
b) If you are the person who ideas are being stolen from, likewise have the honesty to admit that you possibly weren't the first person to ever invent body shots or bondage, and that some "thefts" are truly not intentional.
Now get out there and entertain me, damnit!