They didn't put as much love or thought into all of the maps/specail infected.
This is a stupid statement. The new maps are way more unique and have more variety than the originals, as well as having more interesting features (carnival gnome, carnival games, heavy rainfall, running crescendos, running finale, scavenger finale, persistent levels (hard rain), jukeboxes, the alarm car filled junkyard, the variable paths, witch filled areas in the sugar mill, et cetera)
The new special infected were carefully made to discourage the old tactic of sitting in a corner meleeing and shooting; this is very clear. I'm not exactly sure where to place the jockey but I think it's something to the effect of getting people who run ahead or taking out teams that let their players fall behind.
The new items and gamemodes are just icing on the cake. I think most of the L4D2 hate boils down to
1) it wasn't free and it's easy to write off just how much they added because of how fast it came out
2) The new survivors
3) THE LIGHT IT BURNS