Here's a One Piece FAQ.
One Piece is mainstream Shonen Jump. Hunter x Hunter, Dragonball, Shokugeki no Soma, Bleach, Naruto, Slam Dunk, Gintama, My Hero Academy, Reborn!, Death Note, Assassination Classroom, Kuroko's Basketball, JoJo's Bizarre Adventure ...
What's the biggest obstacle for new viewers?
Its appearance is what makes it unique but it's also the biggest barrier for entry. One Piece's style looks different from any other show, and the strong colors are reminiscent of "cartoons" rather than anime. The author is very free with his art -- characters can be ostensibly "normal" humans but very large, have disproportionate heads/limbs etc. and nobody bats an eye. Keep in mind the world is full of many non-human races as well.
How many episodes are out?
814 as of this writing. Nearly 200 fewer than the "1000" figure people use. One Piece has several filler arcs that are entirely skippable (except the last episode of the first filler arc, which bleeds back into the main story), which reduces the overall count a bit.
When does it start to get interesting?
Episode 3, like most shows. You begin to see the world isn't so clearcut and neither are the characters.
When does it go from good to awesome?
During the first major arc, episode 31 to 44. If it doesn't do it for you by then, One Piece isn't for you. Note it's a bit slow until enough crew have built up. It's a snowball effect of quality as they gather.
I saw some, but it seems silly.
One Piece had some issues like the 2011 Hunter x Hunter beginning. It starts out almost modified to be aimed at kids in Japan, but the blood and severity comes back by about Ep. 16. It's more a quantity tone down for TV broadcast. One Piece isn't dark on the whole, but does deal with tragedy, death, slavery, wars, etc. The author/artist is pretty whimsical, but can also be a bit grotesque.
There was a notorious intentional kid-showifying Dub attempt by 4Kids I believe. They literally replaced guns with corkshooters and cigarettes with lollypops. Also heavy cutting and rearranging of scenes. It was quite a different "show." I recommend watching the subbed "Special Edition" version here on Crunchyroll -- it's a remastered version of the original broadcasts.
Why is there filler?
Almost universally, long-running series have filler. One episode can cover 2-5 (weekly or monthly) chapters of manga or light novel. Assuming a perfect world without breaks, Anime overtakes the manga quickly. Filler gives the manga time without needing to put the anime on hold for months or years.
Should I watch the filler?
One Piece filler isn't terrible, but at best it's entertaining. On the positive side, filler arcs are typically short with no impact on the main story, letting you skip them easily.
Do I need to watch any of the movies?
No, the theater releases are non-canon (filler) or recap-based.
Gintama starts at Episode 3! 1&2 was a promotional TV special.
I didn't really care for it on my first, second, even third try watching it.
The fourth time I tried to watch it (always run out of anime to watch) I started not from where I left off but from the current arc. Happened to be the Dressrosa Arc. A lot of people would say it's one of the weaker arcs but I really enjoyed it. Then I needed to know more about Law, so I had to go back an arc. Then I needed to know about the time skip, back more. All the way back to Thriller Bark before I stopped. Now I jump around the old episodes and stay current.
The show is amazing. The power levels stay consistent from the first episode to 800+ episodes later. The foresight in the series is unparalleled. I love watching it just because Oda is such a master storyteller. To see the giant scope of the story and still be carrying out of faithfully decades later, that's real talent.
I haven't seen more than 10 episodes of Naruto or Bleach. I can compare One Piece to Hunter x Hunter (writer goes on constant hiatus) and Dragon Ball (which power levels are crazy inconsistent) and Yu Yu Hakashu (power levels get way out of wack here as well). I just really respect One Piece. While there are episodes I don't care for, each person has their own favorite arc and character and they are still developing the characters in believable ways hundreds of episodes later. Just a great show