I myself played since the beta version of the game an fell in love with it the second I started playing. It's a cleverly made game and is fairly unique in retrospect to most games on the internet, and the whole game is very creative and interesting. It's a free, high quality game, which is nice and hard to find nowadays as well. It's entertaining and is good for almost any age kid above 10. It's very child-esque but it's not completely baby-fied, making it good for us teens too. It does require you to think and problem solve as well as use logic to complete many of the quests, and it requires patience at times, which does make it a little hard for younger players to understand at times.
Graphics and gore/violence:
It's pretty much completely non-graphic, but it still has nice graphics. There is no gore (no blood, no wounds showing, etc.) and It does not suggest any actual death in the game, in fact, I believe it actually refers to 'defeating' monsters, and you can only be knocked out, and you can't harm other players. Some of the monsters are a bit creepy looking and may disturb younger kids. There are weapons that sort of resemble real weapons, (rifles, swords, pistols) but they all look like toys, and none of them actually cut or shoot bullets. My favorite example is the "T.H.U.M.P.E.R.", which is a microwave powered teddybear launching rifle.
Social interaction, language, and bullying:
You can be social and be safe as the chat censors any word it is told not to show, along with any words it doesn't know. It also seems to censor many of the words that wouldn't be used for anything but bullying. Bullies are actually very rare, and are very well handled when they are found. This they actually seem to care quite a bit about which is awesome.
'Gender' related concerns:
I've heard some people saying that there are "revealing outfits" for the girls... Honestly, I've seen worse dressed 4th graders... The most revealing things I've seen would probably be the halter tops (which are slightly cropped to around an inch below the bottom of the rib cage with a neckline like a cami and covers everything,) the leather jacket that's unzipped to the bottom of the collar bone, and the cheerleader skirts (which have a matching colored pair of shorts under them). There is also no detail to inappropriate areas (flat chested girls, flat behinds) and you literally can not be naked, as it gives you a tank if you take off your top, shorts if you take off your pants, and flip-flops if you remove your shoes. I really think the modesty is quite nice as there are no trashy looking people walking around. Of course, parents may have their own preference on what is acceptably modest.
I haven't had any problems myself, But my sister has. However, they aren't viruses, they are simply some coding patches that were done poorly. It doesn't have viruses on it but it does have A LOT of glitches that could result in a virus. I completely trust the game, but I'm not sure about Unity Web Player, which it needs in order to run. I've seen many Unity based games that have similar problems, but if you have good virus protection like Norton, or Firewall, you should be fine but it isn't completely safe. It is still very unlikely that you'd get a virus from it anyways though. I must warn you, it may cause a little lag on your computer, but again, I believe that's from Unity.
It's a good, quality game for most any age, and gender. It is slightly violent, but has no gore, and has modesty in mind. It doesn't have many bullies, or language issues, and has almost no probability of giving a virus. I give it a thumbs up.