One thing Eve has which helps casual players is their skill training system. You select a skill while logged in, then your character learns that skill whether you're logged in or not. So, you can have character progression while not playing the game (and some of the advanced skills take ages - plenty of time for a holiday
).
Of course, you could argue that this prevents skilled players catching up with more long-term players, but /shrug every system has flaws.
There's also no class-system, so although there are raids, you're not going to have to cancel a raid because, say, no priests turn up. There's skill-based specialisation of course, but for example if your uber-pro hauler-dude goes on holiday, you can do it yourself (doesn't take long to get the basic hauling skills), albeit slower.
Obviously you need to be very active if you want to becoming the CEO of a massive mining/PK/whatever corp. but as long as you find a guild that's big enough to cope without you, activity shouldn't be a problem.
