Tackling is definitely a skill. It takes a mindset of recklessness, relentlessness, and talent.
For instance, Sean Taylor & Ed Reed started out as great HITTERS (ST being bigger and more forceful, which knocked a lot of players out) but you can recover from a big hit and keep running. They then become much better tacklers into their careers.
I talked to a guy from the Vikes a few years ago who learned from maybe the best tackling CB of all time, Antonie Winfiled, who basically said he learned to drives through a player full speed before they can make a move, with the mindset that if he missed he would at least clip a guy who would get tackled by others.
I hate the NFL in the fact that these tackling rules and technique in commerical are not taught on the field in real life, its all for show, and discussed by men who never tackled anybody in thier entire life.