I'm gonna start out with my background in ESO, so if you just want to skip to the actual topic, scroll to the line of underscores.
I have not been playing ESO since beta. I picked it up during a summer Steam sale and started playing it in late July of this year. My first character was a breton templar and I had absolutely no idea what I was doing. I treated the game like Skyrim, assigning attribute points evenly across magic, health and stam, wore green tempered heavy armor, used spells while wielding a greatsword while trying to spam wrecking blow, and hardly did any damage whatsoever. At level 20 I entered PvP for the first time and wrecked this terrible high elf templar wandering around in Chorrol. After that ego boost, I ran to Glade or somewhere and got destroyed by a DK. I got into a few other fights and skirmishes, died very quickly, and figured I was too low of a level to be competitive. After that, I only did PvE until a few weeks ago.
What I've learned so far;
1. PvE encourages bad gamestyles. Granted, not everyone comes from PvE with terrible habits, but if all you do is the quests and side missions until Vet 1, it's not enough of a challenge to prepare yourself for PvP. That's why I was incredibly excited for Maelstrom arena, since it's a good simulator for single players to test their class builds and see if their character has enough sustain and damage. It's not the same as PvP, but it's a better test of ability than saving the leader of your faction over and over again.
2. ESO's terrible introduction to PvP in general. In order for a new player to even have a chance in PvP, they can't walk in with a chip on their shoulder or else they'll rage quit and never touch it again. You either need in-game friends to give you helpful advice, or find somebody to duel with to figure out which skills work and which don't. That's why every time I get ganked and a decent fight ensues, regardless of the outcome, I whisper the other person "good fight" and ask for a rematch. I've made several friends that way, and we occassionally get together and test out builds on each other. This small-scale PvP has spoiled me, because it has so much potential, works very well, yet there's no official support for it from ZOS. I'll get to that later. Aside from the social introduction to PvP, ZOS does a terrible job of preparing a player for any actual encounter. Sure, there's a tutorial on sieging and Cyrodiil PvE, but that's it. This is a big problem.
3. PvP should require an excessive amount of homework. Everytime something gets nerfed, I always picture a teary eyed RPG fan crying over his scarring experience with the mean nightblade who killed him with 2 hits. If that's the case, learn what your class' strengths and weakenesses are. Better yet, learn how to counter nightblades, because counters for every class do exist. If you're a DK, keep up the hard CC, if you're a sorc, keep curse on him, if you're another nightblade, keep up mark, and if you're a magic templar, just run or accept your fate. Seriously though, the only way to be halfway decent is to practice, and I don't mean zerging while spamming one or two abilities, I mean engaging in small scale combat.

Alright

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PvP Dynamics
1. Zergs: We all hate them. They cause lag, they're excessivly rewarding thanks to ZOS, and they're effective. My idea of a fun PvP experience isn't watching a 4fps powerpoint presentation of a group of 40 people spamming proxy det, healing springs, impulse, and steel tornado running through a broken wall in a keep. Not only does this affect the players in the area of the zerg, it affects every player on the server. Sometimes I can feel a zerg coming when my screen starts to skip and my ping spikes over 600, and then I'm hit with a recap screen of how many AoE's simultaneously hit me. The only other option is to try and get together with friends from other factions and find a low population server, and that's if we can even get in due to the ridiculous server swapping policies. I'm definitely not the first to say this, but the zerg gameplay has to stop, and the biggest irony is that ZOS supports the zergs through gameplay mechanics yet can't support it from a performance standpoint.
2. Broken Mechanics: The biggest complaints I've heard and voiced myself involve the broken CC mechanics. Whether it's a delay in breaking it or the lack of immunity after breaking CC, it's a cheap way of getting killed. Similar to the zerg scenario, it rewards players who spam abilities, such as the nightblade who spams ambush or lotus fan to root you until his group catches up. I was actually feared off a cliff to my death, because even though I had full stamina, the CC in fear can cause a delay for players to actually get out of it. It was kind of funny as my character screamed while running over a cliff's edge, but the mechanics require a lot of attention. Also, I understand the difference between soft and hard CC, and I have gotten hard CC'd twice in a row within 2 seconds, which shouldn't happen mechanically.
3. Alternative PvP gameplay modes: Cyrodiil can feel like a job. Get up, go to a keep, repel a faction, repair the wall, wash rinse and repeat. Imperial City can feel like hell. Fight a group of 3 people only to get spawn killed by a mob. Seriously, there's so many mobs and so much other condensed content in Imperial City that it's not even a PvP zone. You can farm tel var stones and get PvP gear without even running into another player. So here's a proposition;
PvP Arenas: ZOS has been bombarded with pleas for PvP arenas for ages, and there's a right way and an incredibly wrong way to do this. On the one hand, a 1v1 arena with a leaderboard is a terrible idea. I'm ok with a leaderboard for vet Maelstrom, but we've already seen the class imbalance in those leaderboards. Just imagine the nerfs and gameplay changes because of a 1v1 PvP arena. It'd be ridiculous. That being said, here are some arena ideas;
-1v1 Sparring ring: No leaderboards, all instanced gameplay in either an empty sparring ring or another ring with a couple line-of-sight obstacles. This will be more than enough for 2 players to group up and experiment on their builds in a lag-free environment. It will also save duelists the extensive 10 minute horse simulator after the end of every duel in Cyrodiil just to get back to their dueling buddy. The primary emphasis on this arena would be instanced, scoreless gameplay that 2 players can voluntarily engage in.
-Group PvP arena: There's so much potential for this type of game mode. Also, I'm suggesting what was originally constructed as a playable demo during the beta for ESO; a small scale group vs group arena, and potentially even have 3 groups against each other. The arena settings can and should be varied to cater to different playstyles and class strengths, but what it shouldn't do is have only one arena in a colosseum setting, that will get boring very quickly. This also might be a long shot, but a PvP town would be amazing, involving running over and through buildings or fighting in streets or alleyways, pretty much what Imperial City should've been but without the daedra and op bosses.

Class Balance
I'll keep this short, it's just a list of the top PvP classes I've seen so far. This is my subjective opinion, btw.
1. Stamina Nightblade: insane amounts of burst damage and great defense if played properly.
2. Magicka Nightblade: cloak, mark, lotus fan, fear, magicka nightblades have a lot going for them.
3. Magicka Sorcerer: a close 2nd to magicka nightblade due to shield stacking and burst damage, along with pretty good sustain.
4. Stamina Dragonknight: stamina builds are stronger than magicka builds simply because of their increased ability to stack weapon damage and stam regen, and if a Dragonknight is played properly, they can wreck anyone.
5. Stamina Templar: some of the best stamina morphs out of all the classes with great sustain and survivability.
6. Stamina Sorcerer: high mobility and burst dps, although not many sorc abilities can be used for stamina builds.
7. Magicka Dragonknight: kind of got screwed over in the most current patch, but I'm not familiar enough with the class.
8. Magicka Templar: I won't give a laundry list of problems with magic temps, but they're the weakest class in 1vX, and sometimes in 1v1. There are some people out there who find really good builds for magic temps, but a strong magic temp is rare due to painfully useless abilities and lots of practice required.

If you survived until the end, feel free to let me know what you think. Like I said earlier, even though I've played the game quite a bit, I'm not as well versed in other class abilities and would like to learn more about them.