It's bad for the same reason that Shea Weber's contract is bad: term.
Although Weber will be a good deal older when his contract expires, Kesler still will be what, 37? when his expires. We probably might see his play begin to tail off as soon as next season. By 37, what will he be? Whatever he is at 37, he'll still be paid almost 7M a year.
Now, for a team in win-now mode; a team that's going to suck in 5 years and thus won't need any cap space then, the term on Kesler's contract won't be as much of a worry. But how about Ottawa? They're not in win-now mode and they're not in good position to try to go for a Cup via trade. They're better off trading Karlsson and attempting a full-scale rebuild. Kesler does nothing to help them now, and taking on his contract is like shooting yourself in the foot.
Believe me, as an Angels fan, it pains me to see that we're wasting away what could be the cornerstone of our franchise in Mike Trout. But look at the moves the Angels are trying to make: they all look like this trade, and look where it's getting us.