This is difficult to answer, because a lot of the things are double edged swords.
For example, I like the current Mez mechanic for the strategy it can bring into fights between certain characters. For example, 2 Martial arts scrappers fighting end up having to care a lot about where to put in eagle's claw and how to use that vital 1-3 seconds when the enemies toggles might be suppressed by the stun afterward.
Unfortunately that same PVP Mez mechanic makes fighting against NPC enemies in the PVP zones needlessly frustrating, annoying, and time consuming. Since one of the things I still do occasionally is spend time in the zones completing zone objectives this becomes a problem there.
There are also times when I do in fact appreciate that diminishing returns is making someone who otherwise might be impossible into someone I can challenge. But it seems as though nearly as often it just confuses the fight with complex and difficult to follow systems that are very hard to understand or predict.
Things like activating your self heal, only to find it didn't heal you as much because the system stealth-calculated your regeneration increase set bonus as an already applied heal and then reduced your powers effectiveness according to some bizarre and complicated series of math equations without mentioning it to you makes it hard to track what is going on, or to try to introduce anyone else to the system.
That said- I enjoy PVP in this game, in it's current form, despite it's flaws. I enjoyed pre-i13 PVP more, however I have adapted to many of the changes that were implemented. I enjoy dueling against my friends, especially when it involves RPVP. In other words, the critical thing I enjoy with PVP in it's current incarnation as well as it's previous incarnation is this: Interaction with other people. What I like is the people, and as long as I have friends there I will continue to enjoy PVP.
It is because of this that I believe much of the current difficulty with the system can be traced more than anything else back to a problem of critical mass of participation, rather than any specific system flaw.
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(Advisory: This may or may not deviate too far from the topic at hand, but I felt it was worth putting here. If it's deemed too far from the topic and needs to be modded out of this particular thread I apologize for that in advance, but I hope it sheds some light on what I mean.)
Good PVP requires a kind of critical mass of people to work. The difficulty with PVP pre-i13 was that the number of people participating was slowly going down over time. It was losing critical mass. The changes they tried to make were intended to help that by using things like diminishing returns to reduce the dominance of the elite and allow more people to participate, thus revitalizing PVP as a whole.
Unfortunately the changes did not have the desired effect, or rather they had far more of an effect on one front(weakening the elite hardcore) than intended, while failing to have much of an effect on the other front(bringing in new blood to compete in the newly leveled playing ground).
The core PVP group of the time felt they had suddenly and unfairly had their system pulled out from under them and replaced with something designed to punish them for the time, effort, and skill they had invested into their characters, causing many of them to leave the PVP scene. Meanwhile, the complex and difficult to understand nature of the new systems governing PVP and the sudden sharp difference between the PVE game experience and PVP game experience effectively created a severe limitation on the ability of new people to pick up PVP and learn to play and enjoy it.
The practical outcome of this was that PVP lost critical mass. With too few people remaining to play, and the difficulty of entering the system for new people now too high, many PVP zones appeared to become ghost towns for long periods of time. And that brings us up to now.
I do not have a magic cure, or instant solution to the PVP difficulties. I don't think as some people do that simply flipping a switch and returning us to pre-i13 PVP rules will make everything shiny and golden and wonderful again. I do think that having threads like this to look at what works and what doesn't is a great thing, and am hopeful that something can come out of it. But if anything is to revitalize PVP, I think it will ultimately have to focus on how to bring people into the system in a way that is easy to understand and conducive above all other things to having fun interactions with others.
What the exact way to do that is I do not know, and whether it's something that can be done within the constraints of everything the Devs have to deal with I also don't know, but I trust them to follow what they think is best for the game.
*deep breath*
Anyway... that's my take. I hope it helps a little.