- Pretty much everything around the game itself. The Starter Sets sucked, the adventures sucked, the marketing sucked, the DDI tools sucked for a long time, the eMagazines... Basically, it's a testament to how good 4e itself actually was that they didn't manage to kill it stone dead - it certainly seemed like they were trying!
- The game made too big an assumption that everything would advance in lock-step. In effect, this too often meant that the game boiled down to "roll a 10".
- Combat was waaay too involved. Too much micro-management of conditions, recharges, and the like. Too many interrupts.
- The marketing sucked. I know I've said that, but it's worth saying twice.
- The rules for jumping are idiotic. Seriously.
- Option bloat.
- The Character Builder pretty quickly became a 'must-have' for me - had I not had access, I would have refused to play. I find that unacceptable.
- Likewise, the game could be run without minis, but it just lost way too much in the process. I find that unacceptable, too.
- While the monster stat-blocks were actually an strong improvement, the "everything is an exception" thing went too far. Using standard spell-like abilities, as in 3e, is actually a good thing - the issue was that the stat blocks referred to them by name rather than reprinting the necessary rules. 4e reprinted the rules... but made everything unique. It is possible to have both.
- No clear statement of what the tiers actually meant.
- No support for the Epic tier.
- Magic items became utterly mundane. This started in 3e, but became intolerable here.
- Healing surges. At the very least, these should have been called something else. "Healing Reserve" perhaps?