Like you, I hadn't been to Donington since MoR days until a few years ago. It's great fun now but a totally different beast believe me. It really is a festival rather than a giant heavy metal gig. You are just as likely to see lads in onesies and superman outfits as you are to see them in Motorhead shirts.
As for day tickets, although I did the w/e last year, I did get some day tickets for Metallica on the Saturday for other people and had no problems. 2 years before I got a day ticket for AC/DC on the Friday, and then decided to go for Aerosmith and Motorhead on the Sunday, so got another day ticket. Had no trouble then either.
As you say, just get in quick you'll get one fine. Also, don't bother with a car park ticket. you have to queue for ages and if it's rains a lot, like last year, you'll just get bogged in and struggle to get out. There's places to park up the road if you don't mind a half hour walk before and after the gig. Looking forward to it already. See you there maybe.
You couldn't buy me with a million babe