Watt was appealing for a number of reasons. He played with great instincts and effort, was strong at the POA, showed great bend and flexibility, and he made plays in a variety of ways - deflecting passes, chasing down runs away, and, of course, sacking and pressuring the QB. He also tested as an elite athlete, which is particularly important for a OLB Pass Rushers/Pass Rushers who start wide. I know that Slimm, CK, and I liked him a lot and ranked better than where he was drafted. Having JJ Watt as a brother mattered, too.
I understand why people were high on Harris, but in general, if you're going to take a guy with athletic limitations early, you want him to be outstanding everywhere else, don't want him to be undersized, and you would really like him to have elite hands. Harris had a number of issues beyond his athletic limitations. He's a skilled and instinctive pass rusher, and he still has a chance to be a good player, but his deficiencies both lower his ceiling and increase his chance of failing to develop into something more than a role player.
Every player has a range of outcomes. People who struggle to with talent evaluations tend to randomly assign a best-case scenario to Player X or a worst-case scenario to Player Y, rather than identifying the correct ranges for each. Of course, everyone misses, but the way you miss will speak to the frequency with which you miss. It's no different than a batter, golfer, or shooter (basketball). The consistent/accurate players tend to have pure/efficient swings/shots - giving themselves higher chances to succeed.
