If I'm at a convention, I feel out of place if I'm not in costume. Even after hours, I prefer to either stay in costume, or at least wear pieces of a costume with my civvies. I mean, I'm not wearing Rocketeer helmet with my pajamas, but I'll go to dinner in my Galactica Viper pilot jacket and boots.
Issues of a cosplayer's size and race will always be a part of costuming. The fact is, the more attractive a person is, the more people will want to look at them, no matter what they are wearing. Most people would prefer to look at a hot girl in a crappy Rubies Slave Leia getup than an ugly girl in a screen-accurate Slave Leia bikini. That's just human nature.
But that doesn't mean squat as to the skill of the costumer. A hot girl in a crappy Rubies costume still has a crappy costume. People who care about costuming will still think "that's a crappy costume." And people who don't care about costuming, like all the newspapers and mass media outlets who do their annual "hot cosplayers of Comic-Con" articles, will print pictures of her because she's a hot girl in a skimpy outfit.
People say, "do you want to see a fat Slave Leia?" because the answer is usually "no." But that has little to do with costuming. You may as well ask "when DO you want to see fat people?" You don't want to see a fat Slave Leia because you don't want to see unattractive people in general. I mean, you don't want to see the ugly, hairy guy in swimming trunks when you go to the beach, either. What I DO want to see is a convention filled with people who look like supermodels in screen-accurate costumes. I want to see girls who look exactly like Natalie Portman dressed as Amidala, and Slave Leias who look identical to Carrie Fisher, age 22, and Batman cosplayed by men who look like Christian Bale, Adam West, or Michael Keaton, but not Val Kilmer or George Clooney, and clones of Lynda Carter dressed as Wonder Woman, and I want to see Tyrese Gibson when I look in the mirror everyday. But that's not going to happen, even if I were to stick with P90X for longer than 3 weeks at a time, because we live in the Real World.
So what I really like to see is good costumes. If the cosplayer is attractive, then great, I get to see two things I enjoy instead of just one. But the number one thing I like to see at conventions is good costumes. And I'd even rather see a not-so-good homemade costume than a Halloween store-bought costume. But even a Halloween costume is better than no costume at all, because at least you're getting into the spirit of things, expressing to the world what you love by dressing up. And hopefully that Halloween costume will be a gateway costume that leads you down the road to true costuming.
I think it's great if you're lucky enough to physically resemble the actor who originally played the character you're cosplaying, but it doesn't mean your costume will be better. There's something appealing about seeing someone who looks like Ewan McGregor dressed as Obi-Wan, but I still want to see the guy with the best Obi-Wan costume, regardless of what face he was born with.
If you love a character, then I think you should cosplay them regardless of how your look. Just because you're black doesn't mean you aren't allowed to love Han Solo. Why should you feel pressure to dress as Lando if you don't like Lando?
I don't have a problem with using makeup, wigs, and contact lenses to cosplay a character of a different race. It is obviously a touchy subject, but generally speaking, use of makeup such as "blackface" has been associated with racism because they were used to portray inaccurate stereotypical, racist representations of African Americans and other races. Contextually, that is the complete opposite of using makeup to more accurately represent a specific individual. There are also racist implications to hiring white actors to play non-white characters in movies, TV shows, and live theatre, because in addition to denying employment to an actor of the appropriate race, it reinforces the notion that largely-white audiences would refuse to watch an actor who isn't white, even when they are watching characters who aren't white. But again, the context of cosplay is the opposite of show business. Cosplayers generally dress as a character to express their appreciation of the character, and there is no limit to the number of people who can cosplay the same character. A Mexican guy dressing as Kato is saying "I love Kato," not "I have to portray Kato because I don't think audiences would accept an Asian in the role." Personally, I find it more offensive to see an Aayla Secura cosplayer with a bad, patchy makeup job and the wrong shade of blue, than to see a white guy with a good makeup job cosplaying a black character.
As a side note, nothing makes me laugh in derision like those articles and posts on "easy" costumes to make. Although some costumes are undeniably easier than others, in my experience any costume is "easy" if you don't give a damn, and no costume is "easy" if you really care about doing a good job (unless you have to money to spend on a screen-used costume... you can't get much more accurate than that). For example, my girlfriend was in the huge LOST group at Dragon*Con last year. Easy, right? A sweaty T-shirt, some torn jeans, and you're good to go. Except for the hours we spent going frame by frame through the DVDs and on the internet, to identify the exact brand and style of jeans, belt, and shoes the character wore. And the hours more we spent on the internet trying to find the belt for sale, plus the hours we spent driving around looking for the items. We spent days on a costume that required no sewing at all. It was among the easiest costumes I've worked on. As we like to say, there are no "easy" costumes.