I think simple and clean lines are important. Some submitters make the mistake of being overly creative in their design. They intentionally stagger the thumbs or use far too many colors. Staggered thumbs almost always look like the page fell apart rather than looking cool, and too many colors is just an eyesore. Two and three colors are typically best. I'd go with four at a max. Any more than that and you're a failed attempt at mimicking Picasso.
I am a believer in minimal content. I hate seeing TGPs with rules such as 20 pics minimum. I think 10-15 pics should do it. I build with 16 only because everyone makes me.

With twenty pics, you may as well drop to your knees and blow the surfer yourself. If he can't get off on 20 pics he isn't getting off on 10,000 pics either. When giving away that much you may as well post a banner that says 'don't bother with a paysite when you can get this much for free'. It's just nonsense. Also, sponsor content would last longer and be useful to more builders if there was lower minimum for galleries. If a single gallery consumes an entire new set, then only the first submitter is going to get to use it a most TGPs.
Content will always be king. In year 4099, content will still rule. Fresh content is the end all of end alls. I can't think of another redundant way of saying that.
Tasty, lively descriptions. I realize that a great many TGPs leave very little space for a description while demanding a good one. What the hell can you say in 17 characters? Christ! I want to see really good descriptions. You know, ones that actually describe the content. I hate seeing things like 'Blonde babe who wants a cock'. Wants? How do you know? Did you ask her? I realize it is an attempt at evoking a certain lustful response in the surfer, but can't you just describe it as 'Busty blonde hottie fingering her pussy' or 'Skinny little whore rubs one off'? We aren't all magicians with words and everyone runs dry. When it becomes a real pain in the balls to come up with four or five words, just scan over a TGPs already listed descriptions. That should inspire you.