People have been complaining about forge for awhile now and after reading and having experience with the game I think i can make a list of the problems:
1) Maps. There are no maps really intended for forge. People can argue all they want about impact and ravine but neither are as good as forgeworld. Impact has so much potential, but the blocks look so ugly and you dont have much creativity on the map. Everything is like connect the dots using tunnels. And ravine is just to small. I would have rather had a new forgeworld then 3 small forge maps.
2) Lighting. Great idea, terribly implemented. It had so much potential but somehow 343 messed it up. First off it should render the lighting at the end of editing or in the start menu. It has to render the lighting everytime i switch from monitor to player and then it takes 3-4 seconds just to render something. It may not seem like much but think about it this way, lets say I build a map in 2 hours, i switch about 15-20 times an hour so lets do some math. 3x20= 60x2= 2 minutes. So if i spend 2 hours building a forge map i spend 2 minutes watching it render lighting. Thats not much time, but its annoying nonetheless. It should be able to be toggled.
3) Ugly. Most the items in this version of forge are just ugly. Impact is the worst offender here as those UNSC tunnels are just....UHG! We all complained about halo reachs forge blocks but those are way better then Halo 4s.
4) Items get unaligned. This is the worst. I spend 3-4 hours building an awesome map then the next day when i go to add something my stuff is all uneven. Then i have to go to every item and put it back to how it was. This is worst when you build flat stuff like a griffball court.
Anyways thats the end of my rant. Most this stuff could be fixed by a TU, and after forge 3.0, this makes me want Halo 5 more and more.