My main criteria when creating magic is simply that something impressive or impossible happens. The 'impossible' criteria can also be relaxed slightly to include the 'very unlikely', so as not to exclude coincidence effects.
I am a fan of effects that are a bit more 'implicit' and maybe only hint at hidden skill. For example, rather than do a bunch of fancy cuts and spin the Aces out of the deck, I'd prefer to gently riffle up the side of the deck, make it look like I am doing some sort of mental calculation, and then name the positions the Aces are at. I don't openly state that is what I am doing, but I want the audience to think something as happened. Of course, there is nothing wrong with the former style of magic, but personal taste means I tend to go for the latter.
The be honest, I've never been that bothered about justifying my effects*. Why am I making the card jump to the top of the deck? Because I can and because it's fun. As long as my audience enjoys it at least as much as me then I don't see anything wrong with that.
Which leads me nicely to another one of my criteria: Do I enjoy performing it? I've heard stories about people performing things such as sponge bunnies, even though they hate them, just because they get a good reaction. I just can't do that. Why should I not enjoy my own magic as well? Conversely, Harry Lorayne once said (and I'm paraphrasing) that 'if your favourite routine fails to get a reaction you should drop it from your act.' I disagree. Yes, it probably needs more work, but dropping it entirely feels like 'selling out' to me. I want to do it, so I am going to do it, and people will either get it or they won't. If it doesn't quite play right, then I might try it a different way next time, but I'm not just going to give up on my own material like that.
Other criteria, I feel, is open to interpretation. For example, practicality really depends on the situation. People who do a lot of walk around cocktail parties are likely to consider anything that involves a table to be impractical. Some people may argue a method is impractical if it involves a set up and a reset. But if you're only doing a set show once a night then does that really matter as long as the effect is good?
Rev
*As a side note, I do think we need to make a distinction though between the 'effect' being justified and the method and moves being motivated. For example, I'm not that bothered about justifying why the ball is about to jump under the cup, but I am bothered about justifying why I am passing the ball from one hand to the other (e.g. I might want to pick up a wand with the hand the ball is currently in).