Having just built myself a skin from the ground up and learning quite a bit about what makes skins (and just graphics in general) flow, here are a couple of things i picked up about my tastes and skinning in general.
my own tastes:
transparent 300s and star particles, transparent hit circles (in some way, either center transparent or just low opacity, clean looking circle numbers, and general cohesiveness are a must. I like small cursors with long trails (cursormiddle style), but that is just personal preference. I play on Hidden 80% of the time so many skins with very thin circle outlines and overly elaborate lighting effects can be confusing. Color combos nice, but not necessary.
Opinions of skinning as a whole:
If you plan to release a skin as your own, don't use the components of others. At the very least, create something similar yourself with your own flair. This is how I have actually learned most of my photo editing... "Oh hey that looks cool... I wonder how i can make something like that" then 30 minutes of googling and youtube tutorials later, I end up with something, but I digress.
Keeping the skin consistent definitely makes is look more "complete" than a hodgepodge of different fonts, animations, colors, etc. Of course making sure that things are trimmed and fit properly can make or break a skin too. If skin elements are being force cropped by osu due to bad sizing, or your elements are running off the screen or under other elements because they are too large. I have used multiple pro looking skins that are ruined for me by improper sizing. (the skin "Life" comes to mind) Seriously, its not that hard to just make sure your skin elements fit right.
Sorry for the eyeful of text but if you want my opinion you will get the verbose version because I am bored in class. ^_^
TL;DR: 300s are bad, (semi)transparent hitcircles are a must, don't use elements straight out of other skins for your own releases, and make sure your shit fits right.