One of the things I really like about older versions of D&D was that there was a scale of difficulty. If you were new or wanted to play a fairly simple class, you could play a fighter, if you wanted complex you played a mage. In-between, you picked a cleric or thief. That said, even the fighters, in the campaigns I played in at least, were almost always able to hold their own against the other classes. I've been playing since 1977 and I guess I've been lucky in never having been in a campaign where the fighters in the group felt second fiddle to the other classes. Yes, there have been times were no one really wanted to play the cleric, but there has NEVER once been a time were at least several people didn't want to be a fighter. There has always been at least one person that wanted to play a thief and a mage as well.
Now having said that, I will admit that our group has added a few interesting moves to the fighter over the years. As I posted in another thread, fighters can burn Fatigue(HP's) to do extra damage if they so desire. The amount of Fatigue they burn determines how many extra die (d6) of damage they roll on an attack. The trade off and balancing factor is they are trading extra damage for reduced Stamina(hp). Burn too much Fatigue doing extra damage and you risk getting taken out by your opponent. It's given a little extra something to the fighters in the group for those that want it. Some players use it in almost every battle, others have never used it once in their characters career. It's a nice benefit that allows fighters to stay simple for those that want it.
Getting back to DnD in general though, as time progressed, things started getting added to all the classes to make everyone happy. In the end though it's made everyone too similar. It's impossible to make every class perfectly equal without removing what makes them unique. Look at computer games like WoW to see a perfect example of what happens when you try and make everyone happy and everyone perfectly the same power. You end up with everyone with similar abilities and almost no one happy. (please don't take the WoW reference to mean any particular version of DnD is like WoW, I am just using it as an example of what happens when you try and balance everything to perfection). The best games I've played in each class excels in certain areas, and it's up to the DM to make sure each player has thier turn to shine. Traps and Sneaking, talk to the thief, Need to take out the nasty orc chiefton, Fighter Joe please step up, and so on.
This isn't to say that only one class can and should shine in any given situation, but each class should have areas where they defiantly outshine the others, even if others can perform said actions as well. I think for a lot of players, everything boils down to combat. They want their character no matter what the class to be on par with everyone else in combat. If your DM runs a combat centered campaign, I can see where this might be at least understandable, but I would hope in a majority of campaigns, combat is just one aspect of whats going on during the course of the characters careers.
For me, what I would like to see in DnD next would be for them to go back to the core of the earlier editions and take a look at what made each class unique and use that for the basis of each class. And then, in the later modules, add in all the complexity that others would like.
I would love for the core books to be something that you could sit down with a new player and a piece of paper and have a character created in 15-30 minutes and give them at least a basic idea of how to play their character as well. The later editions are so complex unless someone teaches you it's just overwhelming. My last 4th edition character I created was 5 pages long when I printed it out. I was like holy sh*t!
tldr: Keep the core classes short and simple for the most part. Add the complexity in in later modules.
