Ok, here's why things feel "samey" to some.
You roll up a first level fighter. You pick out 2 at-wills, 1 encounter power, and 1 daily. You note down a couple of class specific abilities, choose 4 skills from your class list, buy a weapon and some armor, and fiddle with the math until you have a first level fighter, ready to go.
Your fighter suffers a terrible fate brought on by Irontooth. Time to roll up a new PC. "Screw tanking" you think, I'm gonna play a wizard.
So you roll up a first level wizard. You pick out 2 at-wills, 1 encounter power, and 2 dailies (of which you can only have one at a time). You note down a couple of class specific abilities, choose 4 skills from your class list, buy an implement and cloth armor, and fiddle with the math until you have a first level wizard, ready to go.
See what just happened? The steps feel EXACTLY the same. Sure, your powers have different parameters (1[W]+Str vs. 1d6+Int), your weapons and armor is different, and your class abilities are different to fit your role, but in the end, both classes are the same skeleton with different clothes thrown on them.
The feeling gets worse as you level up. No class has new exclusive class abilities to look forward to; just another encounter or daily power at the EXACT same level as everyone else. In 3e (and earlier) classes gained unique powers at different levels (rogues get evasion at 2nd, rangers at 9th) or spells were different levels depending on class (Animate Dead: 5th level MU, 3rd level Cleric). Now? You get rituals at the same level no matter if your a wizard, cleric, warlock, or a fighter with Ritual Caster.
When all you have staring down the pipeline are more/better attack powers and a 1/2-dozen utility powers (most of which are just combat abilities minus the attack roll) The classes seem to blur. Who cares if the daily power you got was Fireball or Flame Strike; they're both Atk vs. reflex cubes that deal Xd6 + stat amount of fire.
It also doesn't help that every class gets better at fighting, casting magic, skill-use, AC and defenses at the EXACT SAME 1/2 level rate. Sure, it makes for easier math, but before the fighter had the best to-hit, the rogue had the best reflex save (by miles, not by +2) etc.
"But Remathilis." you say, "What about roles? Clearly a fighter doesn't share the same role as the wizard, ergo he doesn't share the same play-experience?" True, to an extent. Sure D&D has four roles (and they've always had them, more or less) but while a wizard might be in the back rolling to hit with magic missile and a fighter up front rolling to hit with tide of iron, another lingering element bubbles up:
Each role feels exactly the same. All defenders have a mark. Each mark might do a different effect, but at the end of the day it doesn't matter if your a swordmage, warden, or fighter, your main purpose is to run-up, attack, mark, and repeat. Same with leaders: Every leader has a XXX Word-like power that, as a minor action, gives healing surge + extra hp back. The individual amounts and methods vary, but sure as the sun is hot, if it says "leader" you'll find it has some variant on the Healing Word power in its class list.
Oddly, one role has avoided this straight-jacket: Strikers. Some deal Xd6 damage, some grant 2 or more ability score mods to damage, some just have insane [W]s to each power. A barbarian DOESN'T feel like a rogue because if D&D did anything, its created hundreds of permutations on ways to deal damage. Its also the reason controller doesn't feel unified; it lacks a strong-enough mechanical element (heal, tank, damage) to keep it unified. "Crowd Control" doesn't work well on its own.
Lastly, I originally thought getting rid of different "subsystems" would streamline the game and make it easier to play. Why learn a new mechanic just to play a wizard, psion, warlock, etc? Well, here's why. They played different so the game FELT different. A fighter could be a crafty tactician, or he could run up a kill-kill-kill. A wizard needed careful resource management and a more patient player (at least to be effective). A rogue needed to know the ins-and-outs of the skill system, etc. In essence, they were all little mini-games. Those mini-games are gone, and every class is poorer for them.
I think no one (except the most stubborn h4ter) would argue you could roll up a elf druid an play him EXACTLY as a dwarf fighter, but there is a lot of sameyness in the new "one class frame to rule them all" method of advancement. Classes like the psion (which eschews encounter powers for PPs) earlier would have fixed I think a number of complaints (for example, making wizards more daily-heavy while making fighters masters of encounter-powers).