Gaming site Nowgamer also did an interview on the topic of the newly revealed Mesmer... this one with lead designer Eric Flannum:

How challenging was it to integrate the Mesmer profession into the existing profession set? What specific challenges did you have when making sure mesmers are balanced, yet fun to play?

Combat in Guild Wars 2 is very visual, so we had to find a way to make the conditions and mind games that are so signature to the Mesmer become a part of the battlefield.

It was very challenging to integrate the mesmer into the game, which is why it’s the last profession to be revealed. The Mesmer in the original Guild Wars dealt a lot in energy denial and hexing, so the two most challenging things to deal with were our lack of fast regenerating energy and hexes.

We solved the hex problem by giving Mesmers access to a wide variety of conditions as well as by having some of their illusions replicate the effect of some of the old Mesmer hexes.

For example, the phantasm 'backfire' attacks its target whenever that target uses a skill which closely emulates what the old Guild Wars mesmer hex named 'backfire' used to do.

As for energy denial, we decided that all of the other forms of disruption and control the Mesmer had were sufficient and that we didn’t really need it. I think the Guild Wars 2 Memser still fulfils all of the roles of the classic Guild Wars Mesmer, but does it in a different way.
...

The Mesmer’s Shattering and Mantra skills seem both complex yet highly effective. Can you give us a few examples of how players might use these in battle? On the other hand, how do Mesmers tend to fare in melee and firearm combat?

Mantras have a huge advantage being instant cast in that they can be used while channelling another skill. Since the Mesmer has several channelled skills this really comes in handy.

Shattering illusions is the bread and butter of any Mesmer. No matter which weapon you’re using you’re always guaranteed to have access to AOE damage, the confusion condition, a stun, and a good defense against projectiles.

Since using a shatter will affect all of your illusions at once, knowing when to spread illusions between multiple foes (say to stun several at once) versus knowing when to concentrate them in a single area (say to maximize AOE damage or confusion stacking) is all part of being an effective Mesmer.

I’ve found that Mesmers fare very well in melee and firearm combat. They really rely on movement and trickery to keep themselves alive, so of course a Mesmer won’t just attack you with a sword or shoot at you and will instead use illusion and magic to supplement their weapons in a very unique style of combat. Playing a Mesmer feels very different from any of the other professions in Guild Wars 2.
You can find the full interview here