It's not dumb'd down...really, there's a lot more choice and build selections and a lot more content...plus the devs are really working hard at creating the new endgame and balancing the classes, which is good.
So...to give you accurate details on what has changed since you left, look here (specifically the "Whats New" section of this main page):
http://ddowiki.com/
My thoughts...I think you will find yourself hard-pressed to play and keep up with the new style most players play...which is fast paced....zerging. XP-per-minute has warped the game play style. This happened because many vets started focusing on TRs and ETRs and didn't want to dilly-dally their time away....and with so many TRs to be had in each, who can blame them. Well, this caused most new players to follow suit. And to play that fast, self sufficiency was needed (and now EXPECTED).
If you were around for the level 20 epic content, then the new lvl20 and up content is...different. The old epic quests were more difficult than the new "At Level" epic quests in "Hard" difficulty. To explain...there are now multiple selections at quest entrance for difficulty settings (and XP is more or less depending on what difficulty).
Normal = pretty **** easy.
Hard = easier than the old epics you remember.
Elite = harder than the old epics you remember.
Currently, there is a gross difference between the current hard setting and the current elite setting...so it's a big jump for new players to make. No real grouping tactics nor individual tactics really needed for hard, and both needed for elite, even for the uber builds...though those playing uber builds usually have the metagame knowledge and tactics knowledge and forget they earned those and so think new players should already have that knowledge too...may make for some angst if you encounter that...hopefully you wont.
I'm interested to here what you think if you dive back in. Also unsure what world you're playing in, but if it's Ghallanda, feel free to PM me to set up a time to run a quest or ten :-)
Addeding more with this edit: The first expansion pack is awesome and worth getting, and the second is good/ok but also worth getting.