First, I'd like to state that what I hear described as "player entitlement" isn't always bad. Often its quite reasonable.
That being said, I think these things changed:
1. 2e often didn't have rules for things. 3e did. When there are rules, players base their decisions upon them. When those rules suddenly aren't true, this can be very disconcerting. This will remain true in 4e.
2. 3e had a lot of options for characters. Being the fanboys that we are, we read these options, get excited about them, and want to use them. This leads to conflict with DMs who do not permit them. This will probably remain true in 4e.
3. 3e had an expected power curve. Meeting this power curve required certain magic items of certain strength. DMs had a lot of control over what magic items you received, but not a lot of guidance on how to make sure your character met the curve. This created a lot of potential for friction. This is less true in 4e because there are more guidelines on handing out magic items, and because you don't need as many of them to match the expected power curve.
4. 3e had ways for characters to specialize in particular weapons or gear. This meant that unless the DM provided ways to obtain level appropriate gear of the particular needed type, character abilities became unusable. For example, if I create a spiked chain specialized fighter, I need magical spiked chains. If I only find good magical axes, I am either weakened or denied use of some of my class abilities (feats, for a fighter). This is likely to be true in 4e as well, but is lessened by the rules for disenchanting and creating magic items.
5. The need to plan characters out over time to meet requirements for prestige classes and so on meant that players needed greater control over their character's advancement. This is probably reduced in 4e.
6. The overall culture of the game has shifted. Gone are the days when D&D played a lot like Paranoia: Medieval Edition, and you went through half a dozen characters before one stuck around for a few sessions. Players expect to have a reasonable chance at keeping the same character alive for most, or even all, of a campaign. That breeds greater player investment in their character, which causes them to want more control over it. This is probably still true in 4e.
6. Finally, 2e and 3e were very different. Its not surprising that DMs that grew up in 2e (THAC0 kept out the riffraff!) might feel that 3e bred a new attitude of player entitlement. There was certainly a shift from DM control to player control of the game mechanics- not a total shift, and not an entirely bad shift in my opinion, but there was a change. On top of that, D&D got a lot of new blood with the release of 3e. These new players weren't necessarily accustomed to the "the DM is god, you're just playing in his playground" attitude that used to be a lot more prevalent. Some conflict was inevitable as a result. This is less true with every year that passes.
