The use of any ability in stealth should instantly remove stealth. Getting hit while stealthed should reveal you. Stealth probably shouldnt even exist.
The quickness boon should not exist in pvp. Reacting to telegraphed attacks is the whole point of mmo style pvp, and quickness removes that from a lot of skills.
Mass aoe promotes lazy gameplay. It rewards players for spamming attacks without having to worry about who they are going to hit, or where the enemy player is.
The primary use of cc should be to interrupt key skills, and should require careful planning and execution. Aoe cc should only exist on highly telegraphed, long cd skills.
Chaining defensive skills
Defensive skills like evasion, blocks, invuln should not be so common that players can regularly chain 2-3 of them together.
1 shot builds should be forced into 100% damage, 0% defense. Building a 1 shot character should require the use of all offensive traits and utility. If they miss their burst they should have next to no ability to sustain and disengage.
Conditions and Boons
Boons were originally designed to be short term buffs, conditions were originally designed to be sustained damage over time. In general, boons should not be permanent, conditions should not be bursty. Similarly, cleanses should have to be used sparingly, and corrupts/boon strips should be rare.
Players should be able to see their opponents animations.
Please note: This post is about game design, not balance. I am not mentioning issues such as power creep, specific class balance etc.