Aside from improvements in gfx there are some changes that affect gameplay but it generally remained the same (and bugfixed

).
The most noticeable is adjustable speed of replaying enemy's turn. You can also skip it completely and see the result.
More complicated victory conditions for deathmatch games(it still uses the value of territory controlled)
You can now create a replay of any battle (multiplayer or VS ai) as a separate file and send it someone
In single player mode you'll get a detailed in-game briefing before any mission you play. I mean not a static text+picture, but renderred view of battlefield with highlighting of crucial units and countries, directions of main strickes pinpointed by arrows... e.t.c.
There are also some new hotkeys to cycle through units that haven't yet moved and new warnings at end of turn("this unit can fire", "you haven't used any money at _country_", "")
Sound and music: - a dozen of tracks (.ogg encoded), lot's of “voice action” comments of game events.
I'm writing this without looking at "version changes" doc, but these are most noticeable since concept version.