Background: I've been playing D&D 3.5 since September 2010. In that time, I have played in two campaigns: a ten-week campaign that spanned levels 1-10 and a summer-long campaign that put us at 6th-level (gestalt) and in which no levels were gained. I have also DMed two campaigns: a ten-week campaign that spanned levels 1-10 and a 10-week gestalt campaign that spanned levels 6-20. In that time, I have also managed to
drive myself mad with homebrew, a fact that I mention only because of its relevance to my opinion on D&D mechanics.
Social Aspect: D&D is a fantastic way to spend time with people once a week, and an amazing way to explore ways in which you express yourself. It's good, clean fun--as long as your players aren't of That Sort--and everyone works together to spin a compelling and enjoyable story. Building interesting characters, playing them, discussing them, and exploring the game together is an incredibly rewarding experience.
Mechanical Aspect: One of the big things for me is the diversity of class mechanics. I'm not talking about "wizard casts spell, fighter swings sword." Rather, I'm talking about how a wizard prepares spells in spell slots, a swordsage learns maneuvers and prepares some of them, a psion knows powers and has access to power points, and so on.
Classes working in different ways contributes
hugely to them feeling different, in my opinion. Not only is this nice from the viewpoint of diversity in play experience, it also allows you to focus yourself on behaving in-character. Since your wizard has to prepare his spells in advance, you have to think and plan like the massively intelligent character he is. And so on.
Moreover, the diversity in actual class chassis allows for much more breathing room in writing original homebrew. While that may not be a hugely important consideration from the designers' point of view--after all, they do not directly benefit from us writing custom material--it makes the game mechanics much, much richer to me. For a not-so-humble example, when I decided that I wanted to play a magic-user, but also knew that I didn't like the feel of spellcasters, I was able to base a magic system off of a combination of warlocks and martial adepts. The fact that I can do that--
make my concept happen, even if it happens to not fit into the class structure that has been provided--means that I don't end up banging my head against source books while trying to build a simple-but-interesting fire caster.
Admittedly, this leads to a few hiccups, such as the level of difference between the tiers. I can easily see why homogeneity would be simpler to balance. I'm not contesting that. However, the richness of a diverse set of class mechanics is something that I feel is worth the effort and risk. Part of the reason that I love D&D 3.5 more than any other system that I've been exposed to is that I can interact with the system itself on a much deeper level. I don't have play sessions every night. However, every night, I can, if I wish, throw together interesting builds or write up an interesting ability, class, or what have you.