There are a lot bugs that make PvP seem very broken. Many of these have probably been reported, but i'm going to make this issue with all of them.

Can't start sprinting in air/while swimming:
if you're being attacked by someone, its almost impossible to run away because every time you try to sprint, you can't because the attacker hit you into the air

Sprinting stops when blocking/eating:
if you are sprinting, eating/blocking should only slow you down while you are doing the action, and when you are done you should still be sprinting.

You don't get knocked up when you are attacked from under on a ladder:
if you and another player are on a ladder, the other player can attack you from under and you cant escape because you're always in reach

attacking while sprinting makes you stop sprinting:
If you're sprinting after someone to try to kill him, you stop sprinting every time you attack him making it almost impossible to kill

too powerful knock back on non-enchanted sword:
someone can kill you by hitting you into a corner and constantly attacking you. you cant escape because you keep getting hit back into the corner, and combined with the lack of ability to sprint (like i pointed out earlier) its impossible to get away

If you land on the edge of a block in a lava parkour the server thinks you landed in the lava and puts you on fire.

you stop sprinting when you run into a wall: this makes it hard to run from someone that is attacking you. i thing a good solution would be to make you stop sprinting once you have been running into a wall for two seconds (for example)

Holding shift doesn't stop you from falling off of a block if you are holding shift while landing on the very edge of a block. You can still hear the sound of you landing on the block, but you still fall. This happens a lot with iron bar parkour.

occasional glitchy sprint. Sometimes when you sprint you get caught in a strange sprint-stop sprinting-sprint-stop sprinting cycle. as far as i know this only happens on servers.

attack through walls:
players can use hacks to attack through walls and interact with things that cant normally be reached. This can also happen when servers cancel block break events; players can break a block and quickly interact with something on the other side before it comes back

climbing up 2-block high walls:
similar to the previous issue with the block disappearing for a moment when a player breaks a block and the server cancels the event, players can exploit this to jump two block high walls.

Players sometimes appear to be about one block above/below/to the side of where they actually are. this can mean that the client sees the player in the wall, making it impossible to attack the player. since the other player can still attack you, you can easily get killed.