Not even close to half. Of TOS and TNG I'll lump about half into the
watch category and half into the
skip one, but even most of the skips I don't hate. They're just not ones that particularly appeal to me, but I'll still watch them from time to time. Moreover, I like most of DS9, VOY and even ENT. My watch/skip ratio on those is considerably higher. Episodes I truly hate are pretty few and far between.
Descent makes the cut because it's a remarkably poorly written episode. From the basic plot elements, "let's beam everybody down to the surface to search for somebody, leaving only the people least capable of running the ship behind, despite the fact that the Borg have already attacked twice before," to the most basic dialog, "you killed Crosus. I will make you suffer for that." Just painful. "The sons of Soong have joined together, and together we will destroy the Federation."