Since it's release there's been a good amount of controversy over the Celestial Tournament. Many say it is too hard, too intimidating. Others have responded by saying that it's too easy, that in fact they beat it without any special preparation on their first or second try. While both points of view have their merits, the real problem with the Celestial Tournament is that it's simply not fun, and it's not fun in ways that really expose some fundamental problems with PvE pet battles.
First, I want to address the difficulty. The people who say it is too easy are correct in that if you've already leveled all the most powerful pets in the game to level 25, have a good idea how the pet family system works, and have an understanding of when to use dot's, dodges, speed pets, and hard-hitting moves, then yes, it is easy. If this doesn't describe you, then you will find the Celestial Tournament hard or impossible. Those of you like me, who leveled a good number of pets to 25, but didn't always pick the most powerful pets, or who picked the most powerful pets but didn't level very many of them, should be able to beat the Celestial Tournament, but it may take some time. (It took me four tries.) If you're in a worse position and you don't have at least 25 or so of the specific level 25 pets that the tournament requires, the tournament will be impossible for you.
So to clarify the debate, I believe the argument over the difficulty of the Celestial Tournament has nothing to do with the actual difficulty of the tournament. It is an argument about what type of preparation should be necessary to enjoy so-called end-game PvE pet battle content. Those who say it's easy are correct, it was easy for them. Those who complain it's too difficult are also correct, either they didn't have the right pets or they don't know how to use them well. My own experience is that the number of people who are well prepared for the Celestial Tournament is quite small. Further, because of the amount of effort they've put into leveling pets and learning their mechanics, no PvE content is really going to be able to challenge them. And judging from the forums, and the somewhat dubious claims that some people are putting forth, there are people who would never admit it if some PvE content did present a challenge to them.
So to me, making the Celestial Tournament so tedious is like going back to the BC model of raiding, where every raid has an attunement requirement, and there is no way to catch up on gear. Those who beat the BC raids did get a tremendous sense of achievement. But many still said the content was too easy and clamoured for even harder content, while the vast majority of players never got to experience the Illidan fight except as part of an anti-climatic transmog run years later during a subsequent expansion. When you have such an unfriendly design as the Celestial Tournament on a part of the game that is supposed to be a pleasant diversion, you still won't satisfy or challenge the people at the self-proclaimed high end of the skill curve, and you will shut out and alienate a lot of people who really want to experience your content.
If you've read everything I've written so far closely, you'll have noticed that I've stopped short of calling the Celestial Tournament too hard. I do think it's tedious, frustrating, poorly designed, and not fun. But I also have to admit that I beat it after four tries without prior preparation and without many of the pets that might have made it easier (for instance, no raven or crow for Zao). I expect to one-shot it (or at least two-shot it) weekly from here on out. So, while those who are berating others for complaining about it being too hard could use an attitude adjustment, in the end I think they have a point.
But this brings me back around to the real problem with the Celestial Tournament. It is not fun. It's not fun to have bosses where the entire mechanics of every single PvE and PvP pet battle we've ever had get changed because the bosses that take 50% less damage. It's not fun to use an ability that says right there on the tooltip that it will do 500 damage, and then it hits for 250. It's even worse when you're facing Yu'la and that same ability that the tooltip says will hit for 500 damage does 50 or 100 with his shield up. It's not fun to be facing Zao when he has 800 hit points left and to use ion cannon that says right there on the tooltip that it does 1000 damage and watch it hit for 500, and stun you for two turns while he finishes you off. (Note, this did happen to me, but I knew what was coming, I hit Ion Cannon because I was about to die anyway and I was hoping for a crit.)
It's not fun to be facing Chi-Chi and have him cycle through pets, two and three-shotting all of them because he goes first every time and does so much damage. I had multiple attempts on him where pets only landed one blow because he two-shot them before I went to an undead as my first pet who, even at lower speed, is guaranteed three shots. It's not fun doing 1000-1200 damage on Zao over three turns only to watch him heal all that damage with a single move, or to watch Xu-fu heal for 600 and do 600 damage all in one turn.
Generally, it's not fun going from a model where most pets will last about five turns and you have an expectation about what they'll be able to do in that amount of time to a model where pets will only get one to three moves off before they die.
And as little fun as all of that is, it's even less fun when you reach a point where you've used up all your pets, and because you can't heal or revive any of them, you're going to have to start the whole tedious, unfun process again. And in starting it again, you're very well aware of the limitations you have, that is, that you have the level 25 pets that you have, and you're not sure you can do anything different to do better the next time. It's quite possible you'll go through the whole tedious, frustrating instance again and fail again due to an oversight, a key miss at an important time, or simply not having the pets you need.
To me, the inability to heal was a really obnoxious design decision on Blizzard's part, and an aspect of the Celestial Tournament that those who did well in it haven't experienced. For those of us who took multiple attempts, it makes them seem especially unreasonable and stone-hearted. I can tell you, I personally was furious when I had to start over a third time, and I was getting to the point where I didn't want to go through the tedium of beating the first five fights again (I was stuck on Zao and Chi-Chi from my first attempt on). So, while from an objective point of view I think the Celestial Tournament is not hard enough, given the high frustration level that it provokes, it is too hard.