I've been playing with this patch for some time on the PBE, so the shock value of it all has long since worn off. There are plenty of positives about the preseason patch, but I can understand why some people are feeling negative about it. The two things that were the hardest for me to get past are the vision changes and AD itemization changes. Neither of these are entirely negative, but because so much has changed, it kind of feels like a sucker punch at first.
Vision
The reasoning behind the vision changes is sound, but the implementation tends to leave a sour taste in your mouth. Instead of being able to access vision easily, you now have to be much more mindful of where and when you ward. The worst part about it is being pretty cut off from being able to apply vision almost entirely if you decide to switch trinkets, especially either red trinket. The blue trinket is actually quite a good tradeoff, as it allows you to ward much more safely at the cost of the ward itself being easy to spot and remove. Switching to red trinket means that the only source of vision available to you outside of sightstone is a pink ward, and that still feels kind of awful. It feels like with these changes, Riot is trying to force more teamwork by making individual effort less rewarding.
AD Itemization
It takes a few games to really wrap your head around how much the item diversity has increased for ad champions. While these changes are intended for marksmen, a lot of them have found homes on melee ad champs as well. Probably the worst offender of this has been Yasuo, although right now, I'd rather face a team of Yasuos than try to take on Graves, but that's another topic. What I've seen with these changes is that marksmen now have access to multiple item trees that can help them adapt to different situations; when it's better to build cdr/crit/lifesteal/arpen/ad, there are now a myriad of ways to access those stats without losing much ground. It will be interesting to see how this progresses, and what changes will end up being made as time goes on.