I think the difference is that PVM is all about learning the specific strengths and weaknesses of a target while PVP requires a player to adapt to a virtually endless set of possibilities.
In PVM, for example if you flight the Slasher, he's never going to do anything different or original. You know exactly what his attacks are, how much damage he'll do and how you can best damage him. It's never going to change (unless they publish a change or something). If you fight him 100 times, he'll do the exact same stuff 100 times. In PVM you can ignore certain things and still be successful. If you're a spellcaster you can get away with no melee defense; in PVP if you ignore something major you're toast.
In PVP, you'll face whatever your opponent has come up with. Certainly there are always a few popular templates that will be the majority of what you face; but even then you're facing someone on the other end that is thinking and adapting. And you'll also be facing players with specialized or unusual templates that you must adapt to.
I like to do both PVM & PVP but the challenges are different. In PVM, your challenge is to identify the weaknesses of your target then select a template & equipment that allows you to accomplish your task. In PVP, I'd say the first requirement is to understand and address "the basics". There's a lot to know, but here's a brief summary as a starting point (just off the top of my head):
1. You must have melee defense, even if you're a mage (wrestling or mage weapon), otherwise you'll be toast to any dexer/archer/thrower in no time flat.
2. You must be able to deal with curses/paralyze/sleep etc.; so you either have to have 120 resist, or you must carry apples, crates & potions, or maybe you can use chivalry with 4/6 casting. Again, if you don't carry this stuff you'll be eating dirt lickety split.
3. If you're a spellcaster you must max out your casting speed. In PVM it often doesn't matter much because you can just run away and pre-cast. In PVP, you gotta be fast or you'll never get any spells off (offensive or defensive).
4. You need to understand how you're gonna heal yourself. In PVM it might be OK to just rely on greater heal, cleansing winds or bandages, run away and heal, but in PVP you have to be able to utilize multiple methods to heal in bursts because people will be trying to damage you in bursts. So if you're a mage you must use the little heal & greater heal. If you're counting on Chivalry close wounds you better have 4/6 casting. In all cases you almost have to supplement with potions. Try to combine an instant or fast heal (potion, healing stone, confidence, 4/6 chiv) with a longer more powerful heal (greater heal, cleansing winds, bandage), and learn to time them so you can deal with a variety of attack styles.
5. You need some way to kill your opponent. So, if you're a mage, forget about protection (unless you're in a purely defensive position). You'll never string together enough damage to kill anyone who's prepared. If you're a dexer, you need to find a way to stack together damage even as you miss 50% of the time. In all cases it obviously helps to prevent your opponent from healing with poison, mortal, bleed, etc.
So, if you don't have all of the above, you're a sitting duck in PVP. I'm probably forgetting some stuff, and templates vary of course. But if you're missing any of that stuff, you won't be competitive.
Beyond that stuff, there's still more to consider. How will you deal with being dismounted? Disarmed? Lethal Poisoned? Ganked? There's a reason why all sorts of PVP templates include ninjitsu.
And finally, you have to think and adapt. What's you're opponents weakness, how can you take advantage?