Some things that bear noting: These are all my opinions. A couple people pointed out yesterday that there are fantasy fans who like stories with cute telcoms. Yes, but I don't. All I can try is to tell you why I don't.
Also, usually genius authors can get away with anything, so if you note something I say here that has an exception somewhere because of the goodness of the writing, by all means tell me. However, I think it's a cliché precisely because most people use it without having the skill to write it well, so it still gets mentioned.
1) The bond between two people is rarely a conscious choice. There's some kind of outside force that compels the bond instead. For example, a love spell makes the two aware of each other's emotions, and soon enough they are understanding each other perfectly (see point 2). They don't get to choose who they spend the rest of their lives with? And people call me cruel to my characters.
If it's not a love spell, there's a "natural" understanding between two people, often based on reincarnation or the soulbond thing, which can go away and die a nasty death any time it wants to now. No mention of why two people who loved each other in another lifetime- and sometimes hundreds of years and hundreds of miles distant- should love each other now. They just "do," and compatability of personalities, lives, responsibilities, social status, or, in the most extreme cases, sexual orientation is not allowed to stand in the way. This strikes me as lazy plotting because the author can't seem to work out a way for these two people to fall in love without the reincarnation angle. It's not just a link between two people from a different lifetime, which could function as a way of forcing them to stay together a la the way that being trapped in a cave or cage might. It's a loving link, and it's always the deepest and purest and truest love imaginable. Bah.
2) The relationship ends up perfect, no matter how rocky the start is. Sometimes, as in the case of the love spell thing, it's perfect immediately. The couple are sharing each other's every thought and emotion, and instead of being freaked out or even startled, they start agreeing and laughing and falling in love. It's a honeymoon phase that never stops being a honeymoon phase. The couple never goes through any problems, because how can they have problems when they're sharing each other's thoughts and feelings? (I can think of plenty of ways. See point 3).
Other times, they are freaked out or startled, but they never retain that level of discomfort, or end up friends. No, it must be love or nothing else. Telepathy is so much more intimate than getting to know each other by talking, isn't it? And standing by someone when you know they're frightened is so much more praiseworthy than standing by someone out of simple human compassion.
Lovey-dovey bonded couples make me vomit. Their relationships are static, incapable of developing down or across from perfection, and the author vaunts their bond as deeper than all the other relationships in the story. Here's a hint: "Easy to write" does not equal "deep."
3) There are plenty of problems that would arise if you shared minds with someone else, but they never arise in bad fantasy novels. Think for a moment. Don't you have thoughts that you would never want anyone else to see? Violent ones, sexual ones, ones that could get you arrested, thoughts that you wouldn't have meant a moment later but which would hurt someone else if you uttered them aloud? Now imagine someone able to hear those thoughts, and possessed of the ability to scold you in a way that no one else could hear.
Is that telepathic bond sounding as fun right now?
In the purely empathic bonds, the same thing can happen. We've all felt emotions at inappropriate times or in inappropriate settings, ranging from funerals to weddings to church to the office. We've all hoped that someone won't notice that we're bored or angry or aroused. But with that empathic bond, someone would always notice. It would be the end of privacy.
What makes it worse is that some fantasy authors are perfectly aware of that last fact and exult it. I don't understand. Perhaps all their characters are terminally clingy and can really rejoice over someone hanging out in their heads for the rest of their lives?
4) Communication is not the only thing that makes a relationship work. Silence does too, such as knowing when to back off someone in a foul mood, or just listen to a friend ranting and not interrupt. What about problems that don't depend on communication at all, such as one partner being a traitor? If one fantasy character is a committed loyalist, I can't imagine that she would just smile at her lover working for the opposite side and say, "Oh, honey, why didn't you tell me?" It's not the fact that the traitor is keeping it a secret that's the main problem, after all.
I've heard people argue that if a couple could hear each other's thoughts, they never would have problems because misunderstandings about what words meant couldn't arise. But truth hurts just as often as falsehood, and perhaps more. And if one bonded lover gets angry and slaps the other, the fact that the other understands why doesn't change the fact of the slap. Communication doesn't solve everything except in women's magazines, which are far more escapist than most fantasy novels.
5) There are few or no restrictions on this power. A telepathic human couple can apparently communicate from any distance, through solid stone walls, when one is dying or in pain, and not have their communication be affected at all. If there's nothing in your world that can stop them talking to each other, why in the world aren't the governments snatching them up and using them as unstoppable spies? Not all of the couples can be virtuous or rich enough to refuse such employment.
An empathic bond works the same way. One character can be on one side of the world, and the other on the other side, and still they feel what the other person is feeling. Yet this doesn't limit them in other ways. If one is hit by a blow to the side, the other feels the pain but doesn't crumple up. Um, why? If emotion and sensation translate perfectly across the distance, they shouldn't stop translating because the author doesn't want one character inconvenienced.
All of which is a rambling way to say: I think there need to be more restrictions on such bonds. The distance one is simple and effective to apply, since it could then have a range. The two characters can hear each other very well within, say, a mile, less effectively at two miles, even less effectively at three miles, and so on. An empathic bond could have its direction confused by distance.
I restrict telepathy heavily in my one world where I use it. It can only happen between two people who know each other well, and usually only two at once, since my telepathic race have giant egos and can't meld well with more than one other person in-mind at a time. It's also cut off completely by steel, so a steel skullcap effectively keeps someone from a mental call for help.
6) Make realistic adjustments in society for your bonded couple. Having everyone ignore the bond and expect them to behave just like normal people- for example, having it considered rude if one laughs at a mental comment from the other- is a stupid extreme, because it implies that no one is willing to make allowance for magic. On the other hand, setting up an entire telepathic society that is honored and groveled and worshipped to is equally stupid. What is it about telepathy and empathy that make them so special? If there are other forms of magic in the world, they won't be unique, and they may not even be the most powerful. And if non-telepaths can't understand the bonded couple, as the usual insistence runs, the non-telepaths would have no reason to grovel.
So, fit such bonds into the social structure of your world. What happens when two people bond? Are they immediately expected to get married, move in together, fuck? Does it always lead to sex? Do they get sent to do some task, and what happens if they are older, have responsibilities, have children with someone else, are married to someone else? That is a story I have never read, and would like to. The bonded couple are always free of other partners, and are usually virgins. Idiotic love at first sight meme. Where do bonded couples rank on the status scale, and do people understand if they laugh aloud at nothing in the middle of a party? Do you always invite the members of the couple everywhere together?
This is another idea that could have a lot done with it, just like the telcom idea, but authors are content to rest on their clichés and portray endless rose-colored love-bonds.