I'm using gettingmarried.co.uk, and this allows me to import the guest list, check for dietary requirements - you can set up a tick box or a box for them to type into.
You can also set up a blog, a Q&A section (guests can also enter questions on here), and you can import gift lists I think, although that might be just with Prezola.
I don't think you can send invites from there - I'm still sending paper invites, but you could probably just send an email with the link.
You can set up a password for guests, and separate them into day and evening - so for example I'm giving my day guests one password, and evening guests another, that way they RSVP to the correct part of the day.
This is all on the free version, I don't know if there might be more of what you are looking for on the paid version, but it's a really nice, straightforward site to use.
Hope this helps! X