Is it an issue of every class essentially having similar abilities with a different coat of paint?
For me, the issue was more that
within a class, there weren't a lot of options to individualize how your character played. And feats didn't feel like they added that much customization, either. When my gaming group tried out 4e, me and a friend both made fighters independently of each other and they turned out roughly identical. And we were coming from Star Wars Saga Edition, a game where every class had
nearly infinite customization, to the point that the entire group could make characters of the same class that would all play entirely differently. A huge part of our disappointment was that 4E had quite a few mechanics that had been previously introduced in Star Wars, so we were basically expecting D&D Saga Edition and got something way less fun.
There was also the problem that they fucked up the math badly, so the monsters in first adventure module and first Monster Manual were all
way overpowered, so combat during those early trial games was an absolute slog. Like, you do not feel heroic when every mook has got a higher attack bonus and AC than your fighter and you are constantly taking a beating, with only the constant use of healing surges keeping you upright. It was not fun for anyone but the strikers, who got to do all the damage while the fighters got the shit beaten out of them. Apparently later on they fixed this math, but by then it was years too late. We abandoned it after a couple attempts and went back to Star Wars, and then Pathfinder for our future D&D-esque games.
(MMOs might have based the tank/healer/DPS meta on D&D originally, but when ported back to tabletop RPGs it kind of sucks. It's not fun for anyone but the DPS.)
That said, since the release of 5E, I have come around to believe that 4E didn't get a fair shake. I mean, I still don't have much interest in playing it, but at least it
tried to do something new and fix some of the perenniel problems of D&D instead of being lazy and relying on nostalgia from grognards and brand recognition.
Encounter powers, better balance between casters and martials, level-based bonuses to AC and damage so that you get cooler from your fighting skill instead of being entirely reliant on magic items... these were all good things that worked great in Saga Edition. But missing was the sheer amount of customization available, the move away from daily limits to keep players form having to stop and nap mid-dungeon, and the more defense-oriented balance for PCs that prevented a constant slog of taking damage and healing every round for the tanks and healers, and any sense that your characters were heroes instead of red shirts getting their asses handed to them by every goon they met. Really, I think the last two was what killed it for me. Combat was a slog and everyone who wasn't a striker just wasn't having fun.
I'm not saying don't run it, but you might need to compensate for the math problems that make monsters OP and turn combat into a chore. Maybe make sure that you're using monster stats from later in the game's lifetime after they rewrote them to be less potent. Maybe use monsters that are underleveled for your PCs. There are a series of Expertise feats that grant attack roll bonuses that are basically considered
must haves to compensate for the math discrepancies; maybe just give them to everyone for free so they don't have to blow a feat just to keep up.