WoW certainly had a big effect on the MMO landscape. Even existing MMOs in many cases added in features from it. For example, DAoC, EQ, and EQ II all added easy ways to identify quest givers after WoW pioneered the "!" system (one of the few design elements in WoW I'm aware of that was not taken from a previous game). For better or worse, WoW ushered in some new commandments of MMO design:
1. Thou shalt give me something more interesting to do than standing around grinding mobs to level.
Pre-WoW, quests were few and rarely worth doing in most MMOs. If you did muddle through a poorly designed quest in DAoC or EQ, nine times out of ten you would have gotten better XP and loot by standing in a field killing rats for the same amount of time. If you really wanted to level at a decent pace, the best way to do it was generally to grab four of your closest friends and stand in one spot killing the same mobs over and over again as they respawned. I don't know why that seemed like an acceptable state of affairs to me pre-WoW, but it sure as hell doesn't now.
I'm not by any means saying that a MMO has to have a ton of quests, or any quests worth speaking of, to hold my attention these days. Other options include leveling via PvP (see WAR), Diabloesce randomized dungeons and loot pinatas (see PSO and Dungeon Runners), puzzle based gameplay (see Myst Online: URU, and many of the quests in DDO and The Secret World), or advancement through pure crafting (see EQ II). But if the only reasonably rewarding activity a game gives me is standing in one spot killing gnolls for six hours, I'm not sticking around past the tutorial.
2. Thou shalt not hinder my solo play experience, nor shalt thou make whatever random class I pick a sucktastic soloer without giving me some kind of warning on the character generation screen.
Apart from PSO, WoW was the first MMO I played where any class could solo well. Sure, some classes are monsters at solo play while others are merely OK at it. But the fact remains you can pick any class you like on the character selection screen and have a pretty fun time, whether you decide to play solo or spend a lot of time in groups. That's generally true of most MMOs these days.
Contrast with olden MMOs like EQ or DAoC where the majority of classes could barely solo at all past very low levels. "What the best solo class?" was a really common forum topic, since a wrong choice on the character select screen meant that on any given night you logged you would have to stand around twiddling your thumbs until you could get a group together.
I enjoy playing with other humans, but I don't like being forced to. A game with forced grouping is one where you may was well not log in at all if you can't invest a few hours, and one where you are forced to be social whether you feel like it or not. The latter does not suit my temperament (and really never has), while the former doesn't fit my play schedule at all any more.
3. Thou shalt give me stuff more interesting to do in combat then watch as my auto-attacks slowly whittle down my opponent's health. Yes, even at low levels!
Even before WoW, it used to drive me buggy that at low levels on most melee classes all you did was hit auto-attack and then stand there like a drooling moron watching numbers creep across your combat log. Even at higher levels, it was often much the same save that you'd whip out a special ability once or twice a combat. DAoC improved things a good bit by adding melee attack chains and positional attacks to use while you slowly whittled down an opponent's health. However, WoW was probably the first "true MMO" (I'm excluding PSO here) I played where every combat felt fast paced and dynamic.
More modern MMOs have only taken this feeling to greater extremes. For example, there's nothing quite like flying through the air in Champions Online while raining down fireballs on the heads of your foes (cackling madly at their impotence). Allod's Online, DDO, SWTOR, and others have even gone so far as to dispense with auto-attacks altogether. Thankfully, the days of hitting auto-attack and then going off to make a sandwich are long behind us.
4. Thou shalt not design a MMO with graphics so poorly optimized that it's going to run and look like a turd on any PC I could reasonably be expected to own.
I was able to run many old school MMOs when they came out. But that always involved systems that were pretty well on the edge of my budget. At a time when I had been stringing along an older system so long that I couldn't run many current games at all, WoW looked and ran great on my aging PC. Around the same time, EQ II had far inferior art direction and was barely playable on my rig. That really cemented the idea in my head that there is simply no excuse for a MMO developer to release a game that demands cutting edge hardware to run well. With good art direction even relatively simple graphics can be immersive.
Since then, developers seem to have largely caught on to this. My three year old $700 gaming rig runs SWTOR and The Secret World just fine. Not at maxed settings by any means, but good enough to look pretty. For example, any screenshots you see of SWTOR on this blog are on my normal play settings. The last MMO to really screw this up, Tabula Rasa, I believe would likely still be around if more than 20% of the players that tried it didn't have to run it on minimum settings (where it was foggy and fugly). If a $300 gaming console can pump out astounding graphics in hi def, my PC that costs twice as much and has more than double the hardware specs sure as hell should be able to. If it can't, your engine sucks.
I would count myself among those that believes WoW improved the MMO genre by making it more accessible. I wouldn't automatically dismiss a MMO that breaks one of the new commandments. But any two of them? Yeah forget it, whatever game you are designing is not aimed at me.