List of "Golden Rules"

[Rose]

If you could make a list of rules for deck designing, what are some of your must haves? I will go first...
Must have unified borders.
Must have readable indices.
Must have special/notable AoS.

[Fess]

I would add clearly defined pips. Seen some that just, well... haha.

[Justin O.]

Thematic consistancy is deffinitely one of my musts

[Don Boyer]

This list is a good idea - as long as people realize that they shouldn't slavishly follow any set of rules.  They can be bent or broken, IF AND ONLY IF it serves a specific purpose to the design.  For example, some decks are themed around a different "main Ace" and thus they should have a plain Ace of Spades - the Black Club Gold Artifice deck is a prime example.  Along those same line, a minimalist design might opt for a plain everything, not just the Ace of Spades.

Breaking rules out of sheer ignorance of the rules rarely turns out well.

My rule to add here would be that the edges of the cards have the same appearance throughout the deck on every card.  Anything printing over the bleed line needs to be identical, card to card, or it might as well be a marked deck.

Let's also not forget that one decks rule is another deck's flaw.  For example, a subtle one-way mark might be a great feature for a budding magician, but it's anathema for anyone intending to play poker with it.

[Rose]

My rule to add here would be that the edges of the cards have the same appearance throughout the deck on every card.  Anything printing over the bleed line needs to be identical, card to card, or it might as well be a marked deck.

Yes, that is what I was trying to say when I wrote "unified borders" sometimes I can't write what I am trying to say very well.
Yes, rules can be broken.
I should add that "it must be aesthetically pleasing" though this is relative to everyone's personal taste.

[Collector]

If you could make a list of rules for deck designing, what are some of your must haves? ...

"There are no rules"
 
In reality, they can be only for different categories of playing cards because such categories are conventionally formed by them. Transformation playing cards should have traditionally disposed pips incorporated into some artwork. The quantity of such pips should correspond to the number of approprite card. Changed disposition of pips will mean semi-transformation playing cards. You can't create Pin-Up playing cards by placing photos of museums on their faces. etc.

[Don Boyer]

If you could make a list of rules for deck designing, what are some of your must haves? ...

"There are no rules"
 
In reality, they can be only for different categories of playing cards because such categories are conventionally formed by them. Transformation playing cards should have traditionally disposed pips incorporated into some artwork. The quantity of such pips should correspond to the number of approprite card. Changed disposition of pips will mean semi-transformation playing cards. You can't create Pin-Up playing cards by placing photos of museums on their faces. etc.

Good point.  The requirements of one kind of "card person" won't necessarily be the same as for another card person.  For example, a poker player needs a very different type of deck from a card flourisher - the Handlordz "Dominion" deck is a prime specimen of a deck poker players would not like, especially for people playing games with cards played face-up on the table, such as Texas Hold 'Em or Seven-Card Stud.  If you recall, the original Dominion series had cards without different faces, just different indices from card to card.  At a distance, ace-high would look the same as a straight flush.