Other people will have heated debates about healing surges and defender auras, but I'll probably enjoy the game regardless of the mechanics. It's the odd little fluffy things that get me into a tizzy. For example: In 3rd Edition D&D, Elves had a life span of "over 700 years". In 4th Edition, their life span was shortened to "well over 200 years". Okay, arguably, 700 years is "well over 200 years", but I don't think that's what they had in mind. I'm guessing a 300 year old Elf would be considered venerable by 4e standards.
I'm sure they felt they had some valid reasons for lowering Elven life spans. For one thing, it takes some of the mystique out of entering a 500-year-old crypt if one of the party members can say, "Yeah, I was here when it was built. It was nice!" But what really bugs me is that they don't call attention to it anywhere, at least not that I've found. I haven't came across an in-universe explanation for why the life spans shortened, it was just, "Here's the new rules. Elves live to be about 200." It's almost Orwellian, like they're saying, "Elves have always lived to be 200."
The 4e Forgotten Realms setting takes place about 100 years after 3rd Edition. So at some point during the last 100 years, something happened (probably Spellplague-related) that cut Elf life spans by more than half. Exactly how did this play out? Since Elves used to live 700 years, I'm going to assume the Elf population had an average age of 350 at any given time. That means well over half the world's Elf population suddenly dropped dead at one point, when their maximum life spans shortened to "over 200". This is not the sort of event that would have gone by unnoticed. This pointy-eared holocaust would should made the cover of every history scroll. But it's not mentioned in the "10 Important Facts about the past 100 Years" section of the Forgotten Realms Player's Guide.
Of course, the life span of my own character isn't a big deal to me. I've yet to be in a campaign that lasted long enough for aging to be a factor. But it does matter during character creation. In 3rd, they said that Elves mature very slowly, and that an Elf doesn't reach adulthood until about 100. So whenever I would roll up an Elf character, I would generally make them 90-110 years old, just as my Human characters usually start at around 18-22. But the 4e book says they mature at about the same rate as humans. I refuse to believe that when the lifespans suddenly changed, that they also suddenly started maturing that much faster. Reaching adulthood has great significance for people, with rites of passage and other cultural ramifications. It's hard for me to imagine the Elves just suddenly rewrote that much of their cultural guidebooks when the change happened. Of course, I'm sure a lot of younger Elves were delighted to learn they can drink vodka now.
And what if I'm playing in a setting that takes place between 3e and 4e? We don't know what year this big change took place. Was it a sudden change, or gradual? Not that you can "gradually" reduce a race's life span by 500 years, within a 100 year time span. That doesn't even make sense. When I'm creating my character's personality, I generally have an emotional age in mind, but I really want to know that race's equivalent before I write the number down on my character sheet. So if I make an Elf in a campaign that takes place 50 years before 4e, what age do I pick for a young adult?
Anyway, that's the kind of thing that gets under my skin. Welcome to my psychotic little world.