I am not new to pvp, but im not a veteran player either. I thought i would take the time to list the main things that i consider to be "not fun". These things have made me want to quit pvp most days i play, and while some are a l2p issue, i think others are just poor competitive design.
The least fun: Not being able to attack/find your target.
Stealth and target break. This applies mostly to thieves and mes, but to me nothing is less fun than being in a fight with no reasonable counterplay. spamming dodge while waiting for a deadeye to try and shoot you from 1500 range, or wading through a mess of clones being spammed in your face trying to click on the mesmer is pretty much never fun.
Not very fun at all: CC spam
This is the case in a lot of different competitive games, but loss of character control is never fun to experience. My main concern with this is the power creep of cc skills. With elite specs, it seems that there are just more and more ways to completely lock someone down, since some classes have way more cc than opponents have stunbreaks/stab. While veteran players might be able to recognize and avoid cc skills, newer or the average casual player will likely struggle with it for a while, and making one mistake can result in losing the fight.
Moderately un-fun: Visual Noise/skill effects
In teamfights, things like scourge aoe spam, mesmer clone spam, and holo forge/utility skills add a lot of extra skill and particle effects. This makes it difficult to spot other important skills and character animations you need to avoid.
What is fun: Reasonable counterplay options and high risk - high reward design.
Some classes like power rev, core guard, mender sword weaver, and d/p thief are very fun to fight. Most fights i have against these builds are won by the more skilled player, and you are heavily rewarded for anticipating your opponent correctly. d/p thief is a good example of high risk - high reward. You can be very powerful in a fight, but can also die very easily if you get caught or make a mistake. Mender sword weaver would be the opposite of low risk - low reward. You are very survivable, but will likely have difficulty winning any fight quickly and efficiently.