I have a ton of costumes (a whole garment rack's worth). Almost all are "real clothes" not those costumes that come in packages, but some certainly are costumes, just high quality. Other items are regular clothes just modified or when worn with the other stuff they become costumes. These range from poodle skirt and crinoline, to Renaissance queen, to Grim reaper, etc, plus lots of accessories - assorted sets of wings, various vests, capes and cloaks, lots of hats, etc. I never use a full face mask and always advise others to avoid them as well. Its not that they don't look awesome, but rather a question of being practical - people just about always take off a mask to eat or drink, or because they are hot, sweaty and hard to see through, so mask costumes inevitably become half costumes an hour after you arrive at the party/bar/where ever. half face masks, face paint and prosthetics are all all IMO better options.
My personal rule though is I am never someone else. When I'm a witch, I'm not "the Wicked Witch of the West" I'm Willy the Witch. If I'm a pop star I'm not "Madonna" or "Lady Gaga" or whomever, I'm Willy the Pop Star. Etc. I also prefer scary or creepy or dead costumes, not the currently popular trend of sexy. I appreciate funny costumes on others though.