>How can a romance subplot be not only tolerable but interesting?
Have the characters be interesting and engaging. We don't HAVE to like them, but we NEED to enjoy watching them, even if it's just because we love to hate them.
Make sure the two have chemistry, a back and forth, some reason to actually be interested in one another beyond just the initial physical attraction. Let the romance blossom; let us see the different stages of their relationship, don't just skip to the good parts. Unless the whole story is about the romance, don't make the whole story about the romance - there's nothing more annoying than an awkward romance sideplot that keeps getting in the way of the interesting bits.
There's no one formula, but if you pull it off well, it can be supremely satisfying. Just give us a reason for us to want them to be together and you're halfway there.
>How should power levels be balanced,if at all?
Rock Papers Scissors. Tank Healer DPS. etc. If you have a team, try to have the abilities compliment each other, without one being so superior. Conversely, give the antagonists powers that counteract or frustrate the protagonists' abilities.
Also, be extremely careful with doling out new abilities like candy. Let your characters be clever with how they use their abilities (a flying brick might rip through buildings or destroy floors/walls/stairs to catch a fleeing villain), but don't pull something of your ass just because you wrote yourself into a corner. Characters can get new abilities, but try and make it a natural extension, and not something so OP it should utterly break any fight thereafter.
Jojo is a good example - both ways. Araki is not immune to the asspull (GER). However, when his writing is at its best, it's supremely satisfying to watch two characters with defined stands battling not through sheer power, but cleverly applying their powers to try and outmaneuver the others, like Josuke vs. Kira in Part 4.