Pets and energy
The strangest of all the pet changes is this: pets now have energy meters. Pets don't build energy normally and instead require a pet master to use the new Transfer Energy power to grant energy to pets. Transfer Energy is expensive, but most pets have very energy-efficient attacks, so pets will only slow down their master a little bit (once or twice per fight). Even with lots of pets, Transfer Energy is quite effective.
I was somewhat wary of this change at first, because CO's energy mechanics are sort of dicey. I think that this change alone means that many pet masters will need to respec their characters. In the long term though, I think that the new energy- transferring mechanics work pretty well and are easy to handle (assuming you've already got your own energy under control).
The new interface window
The interface window is definitely a step in the right direction. It shows pet health and energy bars, and clicking on the bars allows you to select a pet to give it heals. The interface also allows players to dismiss individual pets if desired. This is a huge improvement from the manual mouse targeting required on Live.
Unfortunately, hotkeys and/or keybinds were not added to the interface. This means players are currently required to use the mouse to click the interface buttons. This is a real problem for gamepad players, especially since pets now have "special skills" that must be clicked in the interface window. The other big problem, I'll show below.
The pet status bars are clear and easy to see, but the individual pet commands, and in particular the pet special ability buttons on the far right, are extremely small and hard to click. These special abilities are essential to most pets, and they have to be manually activated by the player. If you're familiar with the Champions Online UI, you know that the power activation buttons are a little on the small side compared to other games like World of Warcraft. The pet ability buttons are downright tiny. It takes a lot of mouse precision to hit the right action on command. It's very easy to accidentally hit "dismiss" when you mean to hit "attack."
I think the buttons should be made bigger, and players should be able to bind pet actions to hotkeys. I also think the "dismiss" button should be on the far side of the buttons (it's currently the second "power plug" button), to make it less likely to be clicked on accident.
A little quality of life in the doghouse
Some other great things were added that help a lot. Most pets now fly and keep up with the master using travel powers. Pets also have out of combat health regeneration now, which is a huge benefit. Previously, micromanaging pet health bars was one of the largest time wasters for pet masters. It's good that this absolutely essential change was implemented.
Pets also supposedly take less damage from area attacks, and from what I can tell this is at least superficially true. It's hard to say exactly what changed, but in a few combat tests they did seem to die only when focused down by the enemy.
There were two other things that were supposedly added: more HP and higher damage. Since pets now cost energy to maintain, raising their overall effectiveness (from near-zero) seems pretty reasonable. However, this was not actually changed... yet. All pets have the same amount of HP, and it is very, very low. Pets die very quickly to enemy attacks, although it seems to be overall much less quickly, probably due to the damage reduction from AoE attacks.
Pet damage was also not raised. Some pets, like Munitions Bots, were actually nerfed in damage. The wolf summoned by Command Animals is a notable exception; since he stacks a ton of DoTs, his damage is pretty reasonable. Since I predicted it would be top-tier after the pet patch, I feel sort of vindicated.
The bad stuff
Many pets were completely reworked. Tyrannon's Familiar, for instance, is no longer a portable energy battery. Instead, he grants his user a mediocre damage buff when you click his pet ability button. This is a huge nerf, and I doubt it is intended to be a nerf. Also, his Charged buff stacks expire with time, which was not the case with the previous version. Even if his damage is upgraded, the overall flavor of the previous pet is completely gone.
Many pets, including Summon Nightmare, Munitions Bots, Medical Drones (now Support Drone), and Command Animals only summon a single pet instead of the two previous pets. On top of this, Nightmares and Munitions Bots are much weaker individually. This is doubly frustrating for people relying on these powers.
Some pets, such as most of the rituals and Nightmares, are also radically changed. The loss of key powers from many of these pets, such as Breach for Nightmare, Snow Storm for the Primal ritual and Conflagration for the Ebon ritual. These powers were replaced by weaker powers with cooldowns of several minutes, which completely ruins the effectiveness of these pets.
I've stated in previous patch threads that the devs should not implement patches in this manner. Pets are currently way too weak (weaker than on Live), and testing them is not very fun. If the developers want to adequately get enough players testing pets, they need to make the pets too good. If something is overpowered, a lot of players will try to abuse it, and a lot of information will be generated about what is too good. If something is underpowered, a few people will complain a little bit and the rest of the community just won't bother.
In the case of melee powers, a lot of people already used them prior to the Super Power Pack, so a lot of negative feedback arose and the devs eventually made melee overpowered (then never toned it down). I highly doubt that pets will get the same uproar as the nerfs to Haymaker caused. Hopefully the devs still give pets the same consideration, as they are in a lot worse shape currently than melee powers ever have been.