Basic software architecture
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First, try to focus on making a simple client/server model, where you will be able to:

1. Create a new character
2. Save that character (server side)
3. Log in with the character
4. Be able to chat with others
5. Be able to navigate around in 3D

(Saving a character)
Saving a character can be done 1 of 2 ways either using database or using files.

(Choosing a Network protocol)
In C++ there are two protocols that could be used in creating a network one is TCP and the other is UDP. In my opinion I think that it is best to use TCP because its the best for communicating with client/servers.

There is a more detailed walk through on : http://www.devmaster.net/articles/building-mmorpg/

This is my point of view:

1. Working Client/Server communication
2. Working chat system
3. Working voice chat system (I don't think it's really needed, but some people might like the idea)
4. Working character - basic model (creating, saving, loging in and navigating around in 3D)
*. Bug reporting system - can be even used by developers (so they will get some kind of notes what to fix...
5. Working environment (shaped land, water, textures)
6. Adding game objects - environment (trees, stones...), buildings (cities, villages...)
7. Adding creatures
8. Adding proper models of characters + adding NPCs
9. Adding items (armor, weapons... )
10. Adding spell and abilities
11. Working combat system
12. Working loot system
13. Working quest system + quests
14. Working party, guild, raid... (creating, inviting, kicking, disbanding)
15. Added some events (Arena, Battlefield or whatever) + their system
16. Working dungeon system
17. Some kind of transport (mounts, ships, planes, flying mounts, subway, teleports...)
18. Testing
19. Public Testing
20. DONE

After it's done remember to fix bugs and possibly make some updates - some new environments, new items, new creatures, new quests....

I don't force you to use this list, it's up to you. This is just my personal opinion...