As a means of interesting things to avoid, let me point you to this site: Women in Refrigerators
[For those who don't like great sites, “Women in Refrigerators” refers to the detail that women characters seem to suffer more in comics than other kinds of characters (in fact, the term comes from John Stewart's girlfriend being killed and being stuffed into a refrigerator during his tenure as Green Lantern). For example, look at the women in Spidey's life: Gwen Stacy was killed by Green Goblin, Felicia Hardy aka Black Cat has lost her powers and had her powers mutate, Aunt May has died and almost married Doctor Octopus, Betty Leeds was the victim of abuse, and even Mary Jane was kidnapped more than her fair share of times. They also seem to be depowered, maimed, and killed more often as well.]
Although I would dispute the issue when it comes to the fate of heroines that are art of ensemble casts or have their own book, I really can't when it comes to the girlfriend situation. One of the essays on the site points out that the same happens to boyfriends as well; you can't kill off the main character, so someone else needs to suffer, and that pretty much leaves the love interest and the sidekick. And since the sidekick has to put up with so much abuse, anything bad happening to him just feels wrong. Worse, you're killing off the love interest because you want the visceral effect, and you just can't get that if you kill off someone who isn't important.
So, how do you deal with the problem? You can't keep introducing new lovers and friends just to kill them off, or do other nasty things to them; have you noticed that you can tell who is going to die on TV when some new love interest is introduced? If it happens too often, then your fans will start asking why your hero hasn't committed suicide or at least suffering from intense depression (“Every time I meet someone interesting, they seem to die horribly. I need a drink.”).
The solution is to make sure that you have a stable of interesting characters. You need to make all of your characters interesting, and not just those that will be getting a lot of screen time. All of your characters need to have interesting backgrounds, depth, and a little extra dialogue; they need to have just as much as detail as your main characters, even if you never explore that in the script istelf. It's not wasted effort; it gives you a much better handle on the situation when you write, and that's worth any effort you can give him.
If you do this, then you can spread the pain around; whoever you hit with the damage will affect the reader in the way you want them to be affect: Deep and painfully. Also, because readers associate with background characters more, especially if they have been well-written, you can get the same dramatic effect from one of their deaths as well. I imagine that if J. Jonah Jameson died, you can bet that there would be a lot of people not happy with the writers.
In short, if you want to be taken seriously, then you need to take your characters seriously. Do that, and you really have no limits to what you can do. Don't, and you're just writing the usual, boring pablum; do you really want to about baby food or a rich seven-course meal?
[And, yes, this applies to writers of kid lit just as much as it does to novel writers!!]
[EDIT: I've gotten a couple of comment referring to Kyle Rayner's girlfriend Alexandra "Alex" DeWitt, killed by Major Force in GL Vol. 3, Issue #54, (in 1999) was the dead girlfriend of a Green Lantern to inspire the site (and, in fact, it's a picture of that scene on the current front page). However, I was under the impression that the original Woman in a Refrigerator was John Stewart's girlfriend Katma Tui (who was killed in Action Comics Weekly #601 in 1988), killed by Star Sapphire). Given that a few of the articles predate DeWitt's death, I've sent off an e-mail for clarification...]
