The optimal adventuring party is composed of a Fighter, Rogue, Wizard, and Cleric. You could also read that as the optimal adventuring party is composed of a Tank, Skill Monkey, Mage, and Healer, as plenty of classes fill those same roles without leading to a "sub-optimal" party with one exception. While you can easily swap a Barbarian for a Fighter, a Ranger for a Rogue, a Sorcerer for a Wizard, it's hard to find any other class that does what the Cleric does just as well, and that's a problem.
The first issue is that it means one among your group needs to play a Cleric if you are going to have any chance of surviving. Healing in D&D tends to come from magical sources (see: Cleric's spells) most of the time. RPGs should never force the players into a corner in terms of character options as the entire point of tabletops games is to offer player choice in ways that video games and board games cannot. This wouldn't be so much of a problem if the Cleric's other abilities weren't competing for those same spell slots. They have access to some great buff spells and, at higher levels, some powerful offensive spells, but will find few opportunities to use them.
Beyond their spells, Clerics do not have much in the way of utility. Their martial skill is generally adequate, but, due to their role as party healer, they will not want to be on the front lines. Their ability to Turn or Rebuke is nice, but it only affects one type of enemy. Clerics are not generally known for having many skills, and the unique ones they possess are likely to be knowledge skills, which are of varying and situational use.
And then there's the business of keeping your god happy. This is the part of the character that appeals to me as a player, that you have a direct, if somewhat vague relationship to a deity of some sort. The problem with this is that it's tied to the terrible alignment system, and is almost always punitive in nature. If you deviate from your path, you may be stripped of powers. What happens if you're exceptionally devout? Usually nothing. I think the struggle to maintain one's faith would be a great campaign, but that is likely only to appeal to one of the four characters in the party.
I've moved away from more traditional games in the past few months, and this kind of bad design is why. The Cleric is essential to making the game function well, but no one wants to play it. Being your party's healer is a thankless job (which is great gristle for RP purposes), but, more importantly, it's not fun, which is really the point, isn't it?