Consistent, steady contributions to winning on both ends is boring, I guess.
I think the "boring" comments refer more to the fact that Horford just isn't a game changer, and isn't the type of guy who makes a huge amount of big individual plays.
For example, Horford is not the type of guy who is going to draw double teams in the paint, and create open looks for your shooters. He's just not dominant enough in the post - you can throw an average defender at him, single coverage, and that's enough to make him either miss or pass up the shot.
He's not the type of guy who threatens to make you pay for playing him tight on the perimeter - he no longer has the quickness to get by even average mobility defenders, and he's not one to take a lot contested shots, so if you put a body on him he's likely to just pass it off to a teammate.
He's not the type of guy who is going to dominate you by controlling the boards - Horford has averaged somewhere around 7 rebounds a game the past couple of years, which is woeful. He's capable of having big rebounding nights from time to time, but he's horribly inconsistent in that area of the game - if he gets 14 rebounds one night, he's likely to get 4 the next night. He's just not tough enough, physical enough, long enough or athletic enough to take control of the boards on a nightly basis.
He's not the type of guy who is going to anchor your defense and make countless game winning plays on that end of the floor, the way KG used to and the way guys like Bradley and Crowder still do. Hell I think Amir Johnson is a far bigger (and more consistent) impact on the defensive end. Horford may have a great defensive night one day, but he'll follow it up with a poor one the next night - and he no longer has the agility or the foot speed to cover more mobile big men.
All of the above kinda helps to explain why one might look at Horford as a "boring" player. He just doesn't "make things happen" on the court.
HOWEVER where Horford excels is as a team player. He's generally a pretty good team defender, who tends to find himself at the right place, at the right time, more often then not.
He's a very good passer - one of the better passing big men in the NBA. That used to be a big deal because he could draw defensive attention and pass it out to the open man. It's not as big an asset anymore because he isn't really good enough to draw much defensive attention anymore...but it's still a useful attribute that is valuable to have in a team like ours - that is more focused on team play then individual play.
He's a smart player who has good shot selection. Horford rarely takes bad shots. He plays within himself, and he'll usually only take shots he knows he can make. Unfortunately in this day and age that mostly limits him to wide open jumpers and the occasional post baskets against favorable mismatches...but the fact that he only takes what the defense gives him means that he's rarely (if ever) a liability on the court.
Finally, Horford is actually a very good finisher, inside and out. He's one of the better jump-shooting bigs in the league and rarely misses an open jumper. He's is a really good finisher in the paint, with a nice soft touch around the basket. He doesn't draw fouls often (i.e. barely ever), but when he does he's a good shooter and tends to make his shots.
So even though Horford doesn't really CREATE much for you while he's on the court, he is ALWAYS a threat, and you really can't afford to disrespect him or to leave him alone, regardless of where he is on the floor. You always need to be aware of where he is, and you always have to have a man on him - if you let him catch the ball without a defender on him, he'll make you pay for it almost every time.
In that regard, Horford is a lot like Kelly Olynyk...except that Horford is much more consistent due to the fact that he is a lot more confident in his game, and doesn't have major issues with foul trouble (which holds KO back from being a full time starter). This is why Olynyk (like Horford) always has really impressive advanced stats, despite the fact that he never ever looks like a game changing talent. I really like Olynyk because of that, and I really like Horford for the same reason.
However I do not like paying $30M for Horford, when we already have Olynyk on < $5M a year. The only things Horford is legitimately better at then Olynyk are:
1) Rim protection (which Horford also isn't great at either)
2) Playing defense without fouling
Psying an extra $25M a year for those two things is a bit rich for my taste, but if it's only a 1 year deal initially, and that initial deal is enough to convince Durant to join the team, then you have to consider it.
If it's a long term deal for max money, then I think it's very, very difficult to justify the deal on the hope that MAYBE it might bring Durant here. If that plan fails then you have no Durant, and 4-5 years of Kelly Olynyk, at >$30,000,000 a year.