It's great if the person WANTS advice, but not everyone does. It can also be very embarrassing for someone to be told, no matter how politely, "You're doing it wrong." There are enough resources out there that even if someone can't figure out for themselves how to do it "right" (i.e., to get the most DPS/HPS/whatever out of their character, if that's their goal), they should at least be able to figure out that something needs to change, and then (here's the important part) ASK FOR HELP.

I will ALWAYS make the time for a guildie who wants advice with something they don't understand or want to improve, and if it's something I'm not familiar enough with to give them detailed explanations, I'll dig up a friend or online resource that can (and if it's the latter, sometimes even walk them through it). But you can't expect people to go around telling other players why their style is "wrong." That gets you a bad reputation of a different kind.

So, if you're feeling neglected, start paying attention to potential reasons why. Download a damage meter and check the numbers between pulls. Pay attention to how you may be causing problems for everyone else during instances (refusing to interrupt, not paying attention to a mob you've been asked to CC to make sure it stays that way, body-pulling extra packs of trash because you don't look where you're going, people dying if you're the healer or tank). If you can't spot the problem yourself, ask a high-ranked person or someone you respect in your guild to give you an honest assessment. Then, once the problem has been identified, ask for help in fixing it.

You can't run through life--even a game--completely oblivious to the consequences of your own actions, deliberate or caused by ignorance, and expect other people to gently correct you on the way. Part of being an adult is taking responsibility for your own actions and asking for help when you need it. It's not other people's job to try to "fix" everyone else--especially when they have no way of knowing if that person even considers their playstyle to be a problem.