Well I actually just finished beating Wind Waker for about the 8th time, (yes it's my favorite game in the series) and like all Zelda games I don't think people truly HATE the game, but it does need improvements for sure.
Some main negative points that I find through my playthroughs is one, lack of dungeons. The dungeons in this game were simple at best, and none really had the same puzzle like feel that you get with other zelda games. The puzzles that they did have were easy and short to complete, and there was no backtracking neccesary to complete.
Two, the story. While I think this game had some of the best character development done, coupled by some of the best sidequests in the series, I felt that the beggining was really slow. After you go to the Forsaken Fortress to rescue your sister up until you obtain all three pearls the story is dry. You go forth collecting all the pearls without any real meaning behind why your collecting them. What are they? Who are they for? Why do I need to do this? I felt like a was galavanting through the first few dungeons without any real purpose. Started off slow, however i think it's balanced out by IMO one of my favorite and strongest endings of any Zelda games.
Finally, the islands. While I feel the main islands such as Windfall, Dragonroost, Forest Haven, Outset, etc. were done beautifully, the rest of them seemed naked. I felt like I would go to an Island, get whatever secret is their, and then never go back to it again. While I know every single island can't be as complex as the main ones, I feel Nintendo could've done something to spruce up the somewhat boring feel I got from most of the scattered islands.
With these negatives though, there are many positives! I could list all of the beautiful things i saw with this game, but I'll restrict myself to only a few.
The freedom of sailing! I know I just previously stated that I felt the islands were a bit bare, but that certainly didn't stop me from searching and sailing every single square inch of the great sea. Accompanied by the beautiful music that is the sailing theme, and to an extent, the greatest soundtrack of any zelda game IMO, I never once got tired of sailing around aimlessly discovering new things at each turn. As soon as you get your wind waker, you're free to sail wherever you want to and however long you want to (and believe me I did). No other Zelda game has ever gave me the sence of freedom that I got with the Wind Waker. Whether I was aimlessly sailing just engulfing myself in the beautiful scenery and music that the game offered me, or collecting all the treasure charts, nobody can ever take away from me the pure enjoyment i experienced sailing.
And last but not least, the attention to detail. Each character in this game was beautifully scripted with a great personality. No other Zelda game has ever made me want to talk to every single person in the game to find out what they were going to say. With the hilarious expressions from the enemies to the unique personality given to every character, i felt totally emersed (sorry if that's spelt wrong) in the game and it only made me want to play more to see what was coming around the corner.
Overall while Wind Waker is my favorite game in the sereies, like all games it has its faults. It's all up to personal preference really, different strokes for different folks. I don't think anyone hates it, alteast I've never met anybody that has.
Also as a note, it's funny that you mention how the parrying made the game too easy. TBH I never really thought of that, I guess it makes sence the more I think about it. However I rarley ever parryed and by the 1st dungeon I forgot I even could up until the dark nuts or w/e they're called lol.