Here are the biggest differences from GW2 that I can see:
The most valuable thing you can do in PvP is kill another player. If you could solo kill a lvl 50 player or solo capture a keep that was totally undefended, you would acvtually get MORE reward for killing the player! There will be no "keep trading" between factions. The PLAYER vs PLAYER aspect of... PvP... is what is emphasized.
AoE skills are really great in group v group PvP fights. There isn't a cap on how many your AoE can it (As far as I know), so you can definitely bust down a zerg.
Heals are meaningful and 1 player can keep an entire group alive if he isn't interrupted in PvP. 2 players healing 1 tank can keep the tank alive versus 10 enemies spamming damage on him. This is great for PvP, because smarter and more tactical groups will beat bigger groups.
There are many objectives that encourage small groups vs small groups. You will see a lot of 1v1, 2v2, 3v3, etc's in Cyrodiil, because there are worthwhile objectives to go for all over the map in addition to zergs.
The run back to a battle after you've died is relatively long... I'd estimate respawning at the closest resurrection point and running to the nearest keep to get back in the fight takes at least 2-3 minutes (Depending on things like mount vs walking, etc).
Everyone can stealth. You can do some reeeeally fun hit-and-run tactics, or you can hide by a road that people need to run to reinforce their zerg and cause great disruption in enemy supply lines. Did I mention that a few skilled people can beat larger numbers? I'd say a group of 4 that have at least 2 people capable of heals can beat a group of up to 2.5x their size (So a group of 4 could potentially beat up to 10 unorganized players at once of equal level). I foresee a lot of this going on in Cyrodiil after a month of experiencing it.
There is a life after 50 in PvP! I mean that you hit max level, and start to gain "Veteran Ranks" which increase your power and take a long time to earn (relatively speaking). You will be entertained for much longer with this "treadmill" that will make you want to play more. You'll feel like you're earning something, as opposed to GW2's PvP ranks and titles that start to feel unimportant and "grindy" the higher you go.
There is no need to stock up on supply in order to build siege. The idea was if you ran out of supply, your assault of a keep was forced to a halt. While I actually liked the concept , I decided that it was not very good in practice. People started to turtle up around Trebuchets that were a mile off and sit there firing it for 10 minutes until a keep's wall finally fell over. I found ESO's method of assaulting keeps much better, because the way to break a keep assault is to kill players. Anyone can bring siege to attack a keep, but it takes a while to get back to the fight if you die, so a smaller group can whittle a larger force down. This is much more entertaining than GW2.
It's easy to understand. Run into the world, take buildings, and kill bads! (This is aside from 2 minor things to fix in BETA which are making it easier to tell enemies apart from allies at a distance and also sounds that signify that you hit an enemy with your weapon - sometimes you hit and thought you missed b/c sound isn't distinct enough. They announced they are already working on BOTH of these issues, so I expect them to be resolved by launch)
The PvP world is HUGE. There isn't gonna be any easy way for 1 zerg to defend 2 separate keeps being assaulted like in GW2. In GW2, you would often find a zerg wipe 1 group assaulting a keep, then they'd teleport to their starting area and run and wipe the other group assaulting a keep on the other side of the map... all within like 30 seconds.
Zergs won't be the most effective way to go. Sure, you'll always have a zerg or two on the map, but you can actually be very useful as a small group by taking objectives around keeps, killing players running to reinforce their zerg, or discovering quest hubs and doing really fun PvP-centered quests (Or even killing people at PvP-centered quest hubs!!!).
You're instantly competitive. If you enter Cyrodiil at level 10 (The earliest you can enter is lvl 10), your stats are up-leveled to the equivalent of a level 45-50 char (They are currently tweaking this for balance as I write this). So, you're going to be able to make a big difference anywhere you go right from the start.
I'm sure there are more differences, but TL;DR is Elder Scrolls Online PvP is ridiculously fun, easy to learn, and you're immediately competitive at level 10.
