When I commented on how I dislike unmodified 3.5, I got a lot of comments on how it's the GM's job to modify it.
Well, yes and no. When we're specifically talking about RAW 3.5 or unmodified 3.5 we're talking about the core of the game. That's why we use RAW or unmodified while discussion the actual game. I can talk a lot about all the modifications I've made over the years while running games like Dark Heresy and various Dungeons and Dragons games, but that's not what we're talking about when we discuss "unmodified 3.5" is it?
Dungeons and Dragons 3.5 is a particularly spectacular case when we consider an unmodified or RAW system as well. There are a huge number of market modified systems out there for it already. That's why when I chose to play D20, I don't play 3.5, I play a better alternative. Even if I chose to start with the core 3.5 books, when I'm done with it it's no longer unmodified. I can only particularly think of a single time I ran an unmodified game of 3.5 and it was quite different.
The major differences are of course System Mastery. At it's core, the design team introduced elements of System Mastery designed to reward players to play, die and make new characters. Slowly learning what works and what doesn't. The whole system is based around combat and character traps.
If I had a group that did system mastery, I would for sure love to play RAW 3.5 but as written, it's a harsh game that most people wouldn't actually enjoy.
Character traps are those fun things that seem particularly awesome, but at the end of the day run dry. Several spells can fall into this category as they seem really useful but within a few levels can't do anything to help you. Feats are a big one too. There are a number of feats that can be extremely useful to a 1st level adventurer but quickly turn out to be wasted opportunity. Take for example Toughness. A few extra HP seems like a godsend to the first level adventurer, and with a few bad HP rolls quickly becomes required just to stay alive. After the first few levels, a few good hit die rolls for HP and toughness is a wasted feat. This is why there are 3 modifications I always play with. HP is always 1/2HD+1. So a D8 ends up with +5 HP. This negates the risk of the bad rolls (I believe 4e does this). A bad roll will kill you very fast. Second, retraining at every few levels allows players to rebuild away from bad choices and into good ones (It should be noted that 4e does build this into the core system). Finally is character stats. Stats are very important at keeping low level players alive, letting them be effective but eventually as magic weapons and enchantments appear become less effective. This is why I moved to a 2D6+6 system instead of a 3D6 or 4D6 system. It averages out to the same, but it gets rid of any roll being less than an eight. Basically it focuses the bell curve into the area you want characters to be in.
As for variations on 3.5 there are several systems I enjoy. Iron heroes, Arcana Evolved, and numerous other D20 games.
So as you can see, the idea of an unmodified 3.5 is important to discuss so we can know what to change. In fact, we can even look to say 4e got a lot of these changes right. They recognized that an unmodified 3.5 is a hard game that can be unfairly punishing and fixed it.