5e from hell:
Both core and splats are riddled with errors (splats have enormous powercreep, to the point that playing basic classes is almost useless). Errata is only supplied in the digital version, which is supplied through a paid online subscription only. To prevent piracy, these files cannot be downloaded and are only available in graphical PDFs with horrible resolution, that are only viewable while you are online, and only in a custom piece of shoddily designed software. Think E-tools meets DDI meets 90s TSR legal team.
Every 18 to 24 months the system gets a major overhaul, requiring either the purchase of new books, or complete adoption of the horrible software.
Elves can't be bards, half elves can't be paladins. Gnomes, despite coming in the tinker variety only, can choose to take levels as barbarian. The rules for mixing classes vary for each and every race.
For every action there is a different die-size to roll and it is different each and every time whether you should roll high, low, or should look up the result in a table somewhere (with positive and negative results mixed up instead of being from high to low).
The prose reads like a physics text book (the little of it that there is). The rules read like archaic prose.
Character creation takes forever, with the PC having to choose class, class options ala 2e Spells and powers, race, skills, kits, weapon proficiencies, feats, substitution abilities, and carefully plot out courses to overpowered feat options, prestige classes, paths and destinies.
The first two levels are excessively lethal.
Roleplaying is not mentioned at all in the PHB. Minmaxing is encouraged in sidebars. The DMG states to punish players for roleplaying infractions by withholding XP or changing the characters class. The DMG states to keep the book out of the hands of the players at all times.
Players cannot buy, sell or create magic items. Nevertheless, the level progression all but requires a substantial amount of magic to keep pace with the monsters' stats. This isn't stated clearly anywhere in the rules; in fact, the DMG advocates keeping magic rare to enhance its mysteriousness. Despite this, some freak rolls on treasure tables could leave some vastly overpowered magic items in the hands of the party for defeating a kobold.
Traps are often no save, just die. Only rogues can find them. Disabling them is a pure function of the rogues level.