It was nice because it was an auto attack reset, with a huge attack steroid on it. Also, unlike current Rengar, it didn't inhibit your movement nearly as much. Current Q locks you in place for the cast duration which is why there is a camp of Rengar players that believes it is "clunky" or feels "unnatural." I certainly don't wish to alienate people who like Rengar as he is, nor do I wish to alienate ADC mains (lol). But old Rengar had a satisfying kit, and the biggest issue was always that it was difficult to buff/nerf him without his damage being op/negligible.
I certainly miss the old Q more, which is why I'm happy about this revert. But a part of me misses a few things.
One, the empowered Q was even MORE damage, not an AS increase. I may be okay with this, but I'll have to try it out first.
Two, his passive stacks used to affect gameplay rather than just be steroids. I enjoyed faster MS in brush, and larger jump radius, given I met a certain number of kills. Rengar IMO got the short end of the stick when he managed to win the hunt with kha'zix, but even then, you didn't have to kill every person on the enemy team to fully utilize the passive. I think the current passive is thematically pleasing, but I am very reluctant to want to get those stacks when almost inevitably ONE if not more of the enemy get fed. On top of that, if you decide to take Rengar anywhere other than jungle, you inherently lower your chances of getting those stacks until you're able to roam. ASSUMING you're able to roam in the first place, cause most people you ask will tell you that Rengar is not good in lane under usual circumstances. Too many tanky bruisers in top lane that will out tank AND out damage Rengar in his current form, Mid is #nobrush for the most part and thus no easy engage w/o ult, and bot is usually only pulled off with a thresh support and an enemy support not doing their job well. Also I was never a fan of the disappearing passive stacks. I get this was to stop triple q or something else entirely perhaps, but it made me feel more in control. Now I feel like I have to fill out a checklist to keep my stacks, basically just keep auto attacking stuff as opposed to still be able to freeze a lane (more applicable to top than jg) AND save my stacks for whatever I planned on doing with them in the immediate future.
Three, his ult. This was my biggest qualm with the rework. The introduction of camouflage vs. invisibility vexed me. I totally get that people were sick of a fed Rengar bursting them out of nowhere, and I totally get that part of me is simply obsessed with being on the dealing end of that damage transaction, but when it went from being able to stand next to all of them with none of them knowing, to them being able to see you from a (seemingly to me) huge distance and know exactly who it is that you are closest to and you only have vision of who ever is targeted, rather than everyone in the area, it became immensely unenjoyable for me to play him. In fact for the first 10-20 games I played as him post rework I still had to smack myself on the head and remind myself that they could in fact see me coming. I would always get frustrated and say "How did they know?" only to immediately remember the camouflage mechanic.
Allow me to state that some of these issues go away if the player is extremely good with Rengar, or the enemy is just bad. I'm sure there are those who pull off even mid Rengar with ease, but I am not among those, and I THINK I'm not alone. (No I'm not asking Riot to make Rengar mid viable btw)
All in all the first paragraph is the answer to your question and the rest is ramblings, but I just needed to vent how I felt about this and took this opportunity.