I think it's a fun game, I just think people aren't used to change & you "can't make everybody happy." There's some people that want Halo to never change & all that will happen is a select few will still love Halo & it will get dusted by the competition. There is not a ton to change about Halo 5 for me to keep me happy, but the things I can think of:
1.) The maps aren't great. This is what everybody should be talking about first & foremost, not the aim/radar/etc. 343i has never made maps that are "classic" Halo maps like Bungie did for a while. (although I feel Bungie went downhill after a while too) I don't like how all the maps look like plastic & how they basically are carbon copies of each other. Not enough uniqueness & variety.
2.) People go on & on about the motion tracker in Arena, but I say leave it alone or just SLIGHTLY increases it by a couple meters. I noticed in Free-For-All it can be a pain, but in the other modes it wasn't too bad. The thing that I understand that is frustrating is the weapons in Halo have a bit more range now as far as the Assault Rifle, etc, but they haven't made the motion tracker any bigger really. So you can spawn in a base, start to run out of it & get shot in the back way off the screen because of the increased range of the guns
3.) I think BTB Forge maps were a mistake at first. They should have made like 3-4 really good BTB maps then released Forge maps over time as an added extra
4.) 343i kinda "twists" the truth a lot. They said 15 campaign missions, which there weren't. There were 12 because 3 of the missions you just went up & talked to somebody & the mission was over. They said 6 Warzone maps on disc. Then you find out it's the same 3 maps in 2 different playlists, just alternate versions.
5.) REQ balance, I think they actually did a really good job of this considering all they added to the game, but some further balancing is needed. For instance, the Whiplash Railgun probably needs to be toned down in the number of shots you get with it at 4 REQ. Do the Splinter Grenades also need to EMP a Tank for instance? Those should just do a set number of damage to a Tank or vehicle when it hits them once, that's all so if the vehicle is one hit, it should kill them. Also, I feel like you get REQ a little too fast in Warzone:Assault now. That used to be the playlist I could go into to play something that felt a little more "tactical" like you were slowly tactically taking over a base. Now you get REQ so much faster that sometimes I'm level 5+ REQ at the first base. It just turns into everybody spamming their cards at the last two bases now. I think it did need to be increased, but I think they increased it too much.
6.) Aiming, it's not classic Halo aim so people are complaining, but we're getting more control over it in December. I think it barely needs tweaking honestly.
7.) Lack of Social. They desperately needed two Social playlist at launch, but they built the whole game around Arena/Ranked/eSports. That was a DUMB decision. I'm fine with ranked playlists, but there needed to be a 5 vs. 5 Social Slayer playlist & a fun playlist like ActionSack at launch. All the casuals moved on to other games. Some people couldn't even play with their friends because all their friends were Onyx & they were Gold or the like, it's not fun that way.
8.) Last one, but I'll just say it...even for a hardcore Halo fan the campaign was really average. Too many Michael Bay cutscenes, story that you just wanted to skip, & the campaign didn't feel like "Halo" except for a few of the missions. What made people fall in love with Halo were the big, wide-open levels that were long with Marines by you side getting into large firefights or closer up in your face melee/grenade/gun battles. H5 just felt like super linear Call of Duty style campaign with all the action cutscenes you never get to actually take part in.