I've been making D&D 4 characters all week with the DDI software, mostly because it's a fun exercise, but also because our group is changing campaigns and needs a few solid builds. The big difference between 3.5 comes down to race. Race is more important than it seems on first glance. The change comes from the removal of the complex 3.5 bonus system that required expertise in algebra to navigate. Armor class is the best example, as you can't bump it with magic items carefully selected for the many slots available. This makes the game easier and more enjoyable, less of a math lesson, but it puts too much emphasis on character attributes.
Armor class in 4th edition is based on your actual armor, perhaps with a small magic bonus, your modifiers from dex or int if you're wearing light armor, and a progressive bump based on half your level. There are no rings of protection, or even magic shields that provide bonuses, just the basics. The modifiers for armor are carefully balanced to avoid power builds. This means there's a much greater focus on the one area you can boost, min-maxing character attributes, and the best way to do that is to choose a complementary race.
The int or dex bonus in 4th edition is probably four or five times as important as 3rd, because there are fewer options. In fact, after making a dozen or so 3rd level characters, most had the same AC (around 18), while combat builds had something slightly higher (20-22). Armor types initially seemed to have variety, but everyone ended up with the same armor: hide if they could use it and had a high attribute modifier, chain if they didn't, and a feat to gain something better, like plate, if they had a combat build. The same lack of external modifiers and emphasis on min-maxing takes place with combat modifiers like strength or dex, so it's not just AC.
As much as I don't like reading Internet threads about builds, there does seem to be a consensus on the right builds for each class and build, based on the racial modifiers. When new races emerge, the first thing people look at are the attribute modifiers. The new genasi has a +2 int/+2 str, making it the new de-facto warlord build over the eladrin. Some races have duplicate attribute bumps, which means they get a better power, like eladrin (+2 dex/+2 int but with feystep).
This right build issue is troublesome to me. Although I like to power game like the next guy, I've always enjoyed the variety of various class/race combos, something in which 3.5 did a good job. I was happy to come up with a workable build with any race or class thrown in the pot. Racial bonuses were nice, but not critical at mid to high level. I really like the role of race in 4th, that it's not tacked on, that it has a role to play throughout the life of the character. What I don't like is how deterministic it feels. Choosing a non-standard race or build is quirky in 3.5, but seems a detriment in 4th.