As an old school player finally giving D&D 5e a solid go I have quite a few opinions.
The Good
I have to say that I really like how D&D has evolved into a solid ruleset that is 5e. The 5e rules pretty much cover just about anything that may come up in the game. Earlier versions relied upon the DM and players to customize and build upon the somewhat vague rules. This left the game open for tons of home-brewed customizations. Having had to add quite a bit of customization to my Swords & Wizardy campaign of a few years back, I have to say that I like not having to have to do the heavy lifting on finessing the rules and balance of the game. I was able to do back in the day but I was no expert. It involved quite a bit of time that I no longer have. I want to sit down and run a game and 5e has all the rules covered.
I also like that 5e has a lot of flexibility on how you might want to play the game. There is a greater emphasis on role-playing make than the good ol' hack-n-slash. I'm actually having fun as a DM exploring that aspect of the game a bit more. Character backgrounds are a great idea to help players focus on role-playing their character more. I think the races and classes have been well thought out.
The Bad
Getting brand new players into the game is an outrageously heavy lift. For people that have never played RPGs before, all the nuances of the modifiers and rules and if-thens and the level-ups is just way too much to ask of people to enter the game. I'm having that very experience with some new RPGers. I spend the first 20 minutes of a session going over the next layer of rules and ideas and modifiers. Players are slowly getting pieces of it but when the sessions are spaced weeks apart, they don't recall all the various modifiers needed for various situations. I've had to create quick-starts and notes for them. It's a long way from notecards.
Session zero. What adults who have never played the game before have the time to sit though a session zero? I don't and I'm the DM. Time is precious for everyone and spending the first three hours going over rules and how to play is not a very friendly entry point for people that want to just get a flavor of the game. Plus, this is the session where you roll up characters? As a group? New players will most likely WTF the game and move on. Sure, pre-gens could help with that but there is still the issue of trying to go over all the minutia of just whats on the character sheet.
The Ugly
The organization of the Player's Handbook and DM Guide are completely for shit. Faction information is split across multiple books including the starter set rules. Does WOTC expect you to purchase every single product so you can have all the information to run a Forgotten Realms campaign it's fullest? Yeah, I don't want to spend a ton of money on this stuff and then have to store it. I'm a quick and dirty player / DM. Give me everything I need where it is expected to be and I'm good to go. Lets face it, there is a shit-ton of fluff and color in the paragraphs of these books to sift through to get to the important information that you need.
Creating a character is such a bizarre complicated process of back and forth in the PHB that it takes all the fun out of what should be a very fun process. I end up relying on 5e character generators online or at D&D Beyond. As a DM I'm still going to kill off PCs if they do something stupid (or try to anyway) but having a player quickly roll-up a new character during the session is going to be pretty sad.
This should be all about keeping players in the session having fun, not having to have players and DM look 101 things up per game session.
The adventure books follow that same pattern. The organization is not for actual gameplay but to casually sit back and read. Again, as with the PHB and DMG there is way to much flipping back and forth for content that is buried somewhere within the pages.
Okay, this is going to be an "get off my lawn" moment but the old classic modules were utilitarian. Printed on cheap paper, you were able to make notes, scratch-out monster HPs, tear out handouts and artwork, encounter charts right up front. Those modules were made to be used and abused. These new adventure books are designed to sit on your bookshelf and be 'collected'. To make any of these adventure books even remotely usable, I have to rewrite most of it into accessible notes for the DM. It's sad because I do think that some of these adventures are great fun but the time I have to dedicate to be able to run one of these is not practical for a casual campaign.
Don't get me wrong, I'm not a hater of 5e, I do like it as a matter of course. I thought I'd be saving some time and energy with a more polished rule-set than my ol' cobbled together S&W game.
On a side note, I've also been a player in a 5e campaign and have really enjoyed that quite a bit. But I'm an experienced gamer and didn't have to struggle with the rules too much.
We have gotten back to our 5e campaign with some noobs and I will be posting a session summary of that shortly.