I'd say I really don't know quite how to rank things to get to a few condensed points, I'm going to regret my decisions later I think. 😄
Rules wordings. The game is really assinine about this. Punching someone with a fist? It counts as a "melee weapon attack", but not an "melee weapon" or "attacking with a melee weapon". You have to keep in mind whichever kind of attack your spell/ability/feat ties into, and your GM might have to ruin your carefully planned out turn and have you restart your planning. This is not a problem specific to melee, but they're the ones that most often get hit the hardest by this.
Concentration: You can only keep one concentration spell active at a time. Great in some ways, but a hassle to keep track of and makes spells that hold your concentration much less appealing. You will also almost always choose the same non-concentration spells, because it gives you one less thing to remember, which leads to a very much cookie-cutter feel. A cleric will always have Spiritual Weapon handy, it's non-concentration, uses a bonus action and it's a relatively low level spell. Spiritual Guardians is the same thing, use an action to cast, Concentration, does Continous AoE damage that scales with spell slots spent, and slows down enemies for a 10minute period or until your concentration is broken..
Feats: Ties into the one above. You have very few feats, they come at fixed points in the class and not tied to character level, can be used to either raise stats or get a feat. Some feats are just so very much better than others. War Caster gives you advantage (Roll 2d20 and pick the highest dice) on Concentration checks against damage, allows you to use Somatic spells without needing a free hand (So you can have a weapon and shield always at the ready, instead of having to drop your weapon and pick it up from the ground if you want to cast a spell) and let's you cast a single target spell as an opportunity attack. Or you can take Weapon Master and gain proficiency in 4 weapons and +1 to your Dex or Str ability score. Not bonus, score. Yay.
Skills; Are generally all determined at character creation, if you want a new skill you have to use a Feat to gain 3 different skills or tools. You don't dole out points anymore, skills are your ability score + proficiency bonus (That rises with character level), so if something changes after character creation you will have to sacrifice that build you wanted next time you get an Ability Score Improvement (Feat) if you want to learn that skill. Or spend months with a personal trainer, paying him every day.
"Bounded Accuracy": 5E is based around this, essentially all checks and ability scores are "limited" to 30, except when they aren't, so they've limited the scaling on everything. Proficiency bonuses range from +2 to +6, you only apply this bonus to things you are proficient in. So you can't really roll for attributes anymore, because those stat bonuses are worth too much, if you get lucky you can start with +5, so you apply that as a wizard would to Intelligence, the Cleric - devout as he is - chooses Religion as a skill, he has studied under the High Priest of Amaunator - but he will still, statistically, be schooled about his own religion by that Wizard, until level 13 when they will be equals.. Doesn't really help with point buy either, since Intelligence is less important than both Wisdom and Constitution, and possibly Strength if the cleric wants to punch people in the face for whatever silly RP reason. And that "bound" is not very difficult to break in various ways, rogues and bards get "expertise" in certain chosen skills allowing them to double their proficiency bonus, so hitting +10 in a skill by level 5 is far from hard. So you will end up with cookie cutter attributes aswell, 15-14-13-12-10-8, I actually ended up the second smartest character in a group because I refused to use the cookie cutter attributes, with 10.
General encounter balance. The game is "balanced" around 6-8 medium combat encounters per day. If you don't mind that kind of slog, the game will eventually become somewhat balanced, because the casters will be able to do bugger all because they will be out of spell slots since they recharge on long rests, and fighters, monks , rogues and warlocks will become more balanced since they recharge on short rests. My group would never stand for something like this, it'd be a boring non-stop grind, but if you don't do this your casters will have access to all of their slots, so you need to up the ante - and it's easy as all hell to wipe a party this way because the maths get thrown out of whack.
The monsters. 3.5Ed had tables for upping monsters level, 5e does not, so you have to improvise. I wanted my group to have an encounter against a minotaur in a mansion, it'd be an easy encounter for the group. I gave the minotaur a chain-mail upping his AC by two, increased his attack bonus by +1 and 34 more HP to even out the action imbalance. Nearly wiped the group out.
Classes. Certain classes have really been left by the wayside. I felt so bad for the GM's girlfriend that decided to play a Beast Master Ranger...
Magic Items. They're supposedly really rare now, meaning that the only way to really get a magic item is through dungeons since you can't buy a magic sword for your fighter. And your fighter really, really needs that magic weapon, because while casters don't care about mundane weapon resistance/immunity, because spells, Melee Warlocks, monks and shapeshifter druids get "natural" magic weapons after a while, your ranger, rogue and fighter don't ever get something like that. "Oh, look, a golem... anyone wants pizza? I'm sitting this one out"...
... "Hey guys, the pizza is getting... I was about to say cold, but it mutated legs and walked out... Oh, and me and Mindy started a family, and now our daughter works at the nursing home. Speaking of which, she wonders if anyone here needs their diaper changed?" **** me if some fights won't turn into a slogfest, especially at higher levels when casters have to plow through the books to find out the exact wording of a spell, check the AoE range, reference either the drawn board or ask the GM to specify the exact positioning of enemies, double check the action economy if they can actually do what they planned, and then execute. The fighter rolls his three attackrolls, the rogue gets two rolls, back to the next caster... Atleast the old editions had morale rolls for enemies that could cut things short, but why have any replacement rules for that?
Wow, I have to send a kudos to whoever decided on the time between auto-saving time on the forum software, I got 3 hard reboots out of nowhere. Also, sorry for the rantiness Indira, I tend to grow bitter from remembering I've had to play 5e for months while waiting for my favourite game to come up again..