I've been thinking about this for a while, and I think I've finally figured it out. I'm one of the people who love PL, so the people who hate it (used to) confuse me. Anyways, here's my theory, haha:
Excluding noobs, who get to level 13 and then are like WUT I HAVE TO PAY THAT'S DUMB I HATE PL, people who don't like PL don't like it because they have a different perception of what they think it should be, not what it is. Since these people have played other MMORPGs, when they play this one they think, 'oh hey, it's an MMORPG, so it has to be this, this, and this.' Then, when it doesn't include those things, they think, 'this is dumb,' rather than 'maybe PL isn't what I originally thought.' Just like with anything, when people have stereotypes or ideas about something, and when that something doesn't fit those stereotypes, those people don't like it because they're used to those stereotypes. So because PL is such a good MMO, people forget that they're playing a mobile game in a medium where games are usually meant to be a distraction for a short time, not a full, in-depth, hours-at-a-time game (granted, there are some exceptions, but most apps fit the 'quick fun' mentality). The PL devs (who are amazing, btw) blended the two in an amazing way, in order so that PL is really fun in short bursts, and fun for a long time. So those people who have played in-depth computer MMOs play PL and say, 'hey look, a really good MMO. since it's really good, it has to be super in-depth because that's what I'm used to.' Let me get this straight: PL has tons of content, but it's not a WoW, because it wasn't designed to be a WoW. It was designed to be the best pick-up-and-play MMO on the app store, and it blew that out of the water.

So it all depends on your perspective: if you come at PL thinking it's going to be a super extensive in-depth game like a computer MMO, you're probably gonna go away thinking it didn't meet your expectations. However, if you come at PL with a fresh slate (as you should, considering it's a mobile MMO), not knowing what to expect, PL will blow you away.

Cheers, mates. Happy campaigning.