The rifts (portals) in Aion work as temporary access points into normally safe enemy homelands. These rifts spawn on a timer and appear at a location chosen randomly from a list of about 14. There's between 2 and 6 up at any given time, they are one-way and remain for around 30 minutes. There's a limit on the level range that can go through and there's a cap on the number of uses the rift has, to prevent zerging through it. On the other side you arrive in one of the enemy's PVE zones, where you are sometimes greeted by a welcoming party, sometimes not, it depends how quickly word spread about the location. From there you can fan out and maraud around the enemy's homeland until you are killed. It's quite fun, but generally the PvP style is ganking players who are engaged in PVE so it's not much of a challenge. Sometimes though some interesting hit and run and chase fights can develop.
The other PvP zone is the Abyss. Which is like Darkness Falls but without any control mechanism. Because it's a PvPvE zone, its full of mobs and so most of the PvP action that goes on there also involves ganking players engaged in PVE. And because there's no level restrictions and the mobs are grouped onto different floating islands with tight level ranges (which you can fly to without engaging any mobs) it is all too easy for high level players to know exactly where to go to kill lowbies. And they do, 24/7.
And that's Aion PvP in a nutshell, a gankers paradise. If you didn't much like the ganking on WoW's PvP servers, Aion is much much worse. There's no sandboxed PvP (along the lines of DAOC or WAR) of any kind. And because players can easily and quickly look your character up on the web via the Aion Armoury, you can be sure that when someone does gank you they already decided they can beat you. Zzzzz.
Add to that the huge grind and the fact that the game is currently totally dominated by bots and gold sellers who are killing the economy and I would say Aion is a game to currently wait and see how it pans out before jumping in. If they fix the grind, ban the bots and widen the appeal of the PvP then the game could have potential. But there's seems no committment from NCsoft to do any of these things.
As for casual friendly MMOs, CoH and WoW are the two that spring to mind. As for Daoc, these days levelling is a breeze, they've made it much much faster (probably faster than WoW even). I've just re-opened my US DAoc account because they've made all expansions free for download and merged all the servers into one single mega server which currently has 4k users prime time. I started at the weekend and it was a real blast, lots and lots of RvR action everywhere, feels like everyone is having one last hurrah because of disappointment with WAR, Aion, WoW etc. And actually with all the expansion packs installed the graphics aren't half bad either. The UI is showing its age though. I've got a 10-days 'come back for free' period. I'll let you know how it goes and if I decide to subscribe at the end of it.