I'm still fairly new to the PvP scene (not ashamed to admit 36 wins to 40 losses), but already I'm disgusted by the behavior of the vast majority of PvP trainers.
First of all, time-stalling. While some see this as a simple strategy, to me it just seems spiteful. Time-stalling can refer to waiting until the timer hits a very low number before you submit your team or take your turn. While sometimes this is simply because players are indecisive about how to act, many arrogant players do this simply to frustrate their opponent in the hopes that it will cause them to miscalculate their turn or, in the case of team selection, scare the other player off (the victim may think "oh crap, he/she put a lot of thought into their team; they'll probably beat me easily!") I think this is quite an underhanded way to do things, gaining the upper hand with no skill or strategy involved. People also time-stall in spite - if they know they'll lose, they may resort to forcing the other player to wait as long as possible for their victory. Time-stalling is unsportsmanlike, spiteful and generally unfair on your opponent.
Next up, rage quitting. In a nutshell, this means disconnecting from a hopeless battle rather than conceding defeat and forfeiting. While there is now a penalty for this, people still do it. I myself have been a victim of this very often - The opponent knows they can't beat me so they disconnect so I don't get the win. I and many other fair players have no time for sore losers in our battles. Disconnecting rather than forfeiting is cowardly, and perhaps surprisingly the players who do it the most are the veterans - they simply can't accept that they can be beaten. Big news - you can. Deal with it.
Lastly, the general disrespect among PvP trainers. If somebody beats me, I'll admit they were better than me and deserve the win - "good job buddy, this one's your's". If I am the victor I still congratulate the loser - "I won, but you put up a good fight!"
Sadly, it seems no-one else shares my etiquette.
The PvP community nowadays seems to run on the basis of "I win, you suck." I have always hated this vibe in other PvP games: every time I'm beaten I imagine the person on the other end laughing and saying "what a noob!" Pokemon is no different. People need to learn to respect their opponent and the fight they put up, rather than gloating. It doesn't matter how easy your victory was (I once lost a battle in 4 turns), you don't belittle your opponent.
That's about it. Think about my rant and if it applies to you, SORT IT OUT.
Thank you.