The number one feature of any mouse, I think, should be comfort. This will vary with every player, and there isn't one mouse that will fit all players. If you plan on playing for extended periods of time -- and World of Warcraft PvP can be an addictive, time-consuming endeavor -- you need a mouse that feels good in your hand and works well with your handling style. Some players rest their palms on their mouse, while others use only their fingers and wrist; some players play with low sensitivity, necessitating long, sweeping motions, and others play with high sensitivity and minimal mouse movement.
The bottom line is that your mouse works as an extension of your dominant hand, moving more than the hand on your keyboard. Find something that feels comfortable for you, although players can always adapt to the mouse they've chosen. If you're used to an ergonomic mouse, for example, you might have trouble adjusting to ambidextrous mice. But the truth is that you can play with any input device that your computer can detect -- I actually used to play (and PvP!) with a Wacom tablet, which made it virtually impossible for anyone to play on my system.
That's the thing: it's your system. Just as your characters are your own, tailored in the way you want from appearance to talents to gear, your gaming system -- if you happen to have a personal one -- should be tailored for you. Want to play with a trackball? A joystick? As long as your computer recognizes your device, you'll be fine. The little details will come into play later.
In the end, I found that my stylus simply wasn't enough for my gaming needs. At some point, your play style might demand specific features that go beyond the capabilities of the generic two-button mouse. In my case, I simply wanted to have more buttons on my movement hand for macros and other abilities.
Other relevant mouse features include CPI or DPI, programmable macros and profiles, or even mouse Hertz. Many of these features will be arcane to the average WoW player, and probably won't even matter unless you're a hardcore gamer with exacting standards. We'll talk about that a little later. But all I wanted, for example, was something with more buttons.
I settled on a Razer DeathAdder, whose five buttons were a tremendous upgrade from my two-button, scroll wheel-less stylus. It was excellent, felt good in my right hand, and served me well for about a week until I decided I wanted even more buttons and traded up to the Razer Lachesis, which has nine.
At that point, comfort was superseded by my need for more buttons, as the ambidextrous Lachesis took more getting used to the natural feel of the DeathAdder. Over time, I became more comfortable with the Lachesis and it became easier to press all its symmetrically-placed buttons. With enough use, you will learn to adapt to your device of choice.
Many players will point out other ergonomic mice with an equal or superior number of buttons. Some of you will note superior features, better technology, etc., and that's alright. There'll be great points all around. Personally, I'm a Razer fan because ultimately, I think it's sexy. At the end of the day, beyond all the technologies, it's a matter of preference.
For example, I like the understated, minimalist design of the Lachesis and the DeathAdder before it. I cringe at the fantasy styling of the Steelseries World of Warcraft special edition mouse, although I'm certain it pushes a lot of other players' buttons. Others will love the Logitech G9 because it reminds them of KITT. It's up to you.