Technically (if you want to get in to it), a plate that has a year letter and then either the numbers 1-20, or repeat numbers (ie 444) is classed as a 'personalised' plate. You could argue that numbers like 90 or 300 would also make it slightly more desirable, but I have a reg value guide book that only lists 1-20 or repeat numbers as making the plate worth more.
If there is no year letter, then it is a 'private' plate, and worth the most.
If it has an I or Z in the letter grouping, along with no other year letter and just numbers, then it's an irish plate, and technically private, but worth a lot less (they start at around £80 I think)
If it's a Q then it's a kit car or something similar (normally when the original VIN plate doesn't correspond to what the car is or soemthing like that, or scrap cars where a certain percentage of the car is newer, not too sure on Q plates)
Of course if a plate is appropriate to a certain person, then in their eyes it is worth more. I know someone who brought the plate ??03 OCT as the ?? were her initials, and her birthday was 3rd october. To anyone else it could just be a standard new plate, but to her it's personal and worth more.