From what I have seen around (on boards and in-game chat) none of the servers have really died off. Most are quite top-heavy now though as they have made the level grind a bit easier, and the cap has been unchanged for nearly 2 years now.
I've played a little on MR, and there definately was a bit of a newbie/lowbie population there. Of course, they could have quite well been alts of high level toons, but they didn't particularly seem so. This was also in the wee hours of the EST mornings, so that may have made a difference as well.
Like chak said, it depends on how long you've been away to know what features are new. Adding to the few he mentioned (maps and charms are from Legacy of Ykesha, AAxp is from Shadows of Luclin, Level 65 cap is from Planes of Power) there have been quite a few new items added.
The most important features in my mind are:
Required upgrade to Dx9. This hit us in March or April of '04. DirectX 9 is now required to play the game.
The new UI (well...newer than the old one...between SoL and PoP this came out) is great. Fully customizable and skinnable.
New (working) Tutorial came out. Quite nice looking and you get a decent charm out of it.
Newbie quests galore. Most classes/races now have newbie armor quests. Gone are the days of being proud of full banded at level 17 =( (/mourn Amerak...my first warrior).
Discipline change. Combat abilities are now hotkeyable and go off different timers. Endurance (new stat similar to mana) is used for these abilities. Replaces the old yellow stamina bar. (Also renders all the stamina regen spells, i.e. the "zings", useless).
Many older zones have been revamped as well. Acrylia Caverns, Plane of Hate, Grieg's End, Droga, Nurga, Charasis (Howling Stones) to name a few (Cazic-Thule too if it's been *that* extended a break ;).