there is a monthly fee well its kind of a monthly fee you pay as you play.
300 hours = £20 ( about )
and if you play for 200 hours and you take a break those 100 hours will still be there when you come back.
Sounds a bit too expensive to encourage migration from WoW in anything like decent numbers.
It is if you expect people to drop WoW for Aion. At that price point all you'll get is the very jaded WoW players who would switch regardless.
If the price point was lower than WoW you'd get more people to try it out.
That said I don't know what their initial offering is. If you get a good few months free when you buy the game then it won't be much of an issue.
Some of the older players (particularly those who remember Daoc) may well consider swapping from WoW (if they play it) to Aion though.
I thought DAoC was a newer mmo ...
see that's the trouble there are many like me who probably would like to play 5-8 hours per month and would pay by the hour but don't want to play every month some months I could play 10-20 hours then other months it would be as few as 8 hours.
Paying 10 quid for say 100-150 hours would be great.