Proposed Addition:
If a resource pack is to be at all effective on a server it needs to be mandatory. currently players can just refuse the resource pack then it will break a lot of the custom content added by your pack.
Additional when adding a RP to your server it can be challenging to decided which definition to use as players may not have a good gaming computer and may not be able to handle the same graphic you can.

Specific Features:
I propose a new settings for server.properties

force-resourcePack=true

And changing the syntax of resource-pack so that it accepts multiple inputs

resource-pack=http://example.com/HD_64x64.zip-http://example.com/Default_16x16.zip

Justification:
It would allow server owners the same power mapmakers when creating rescource packs for their Map/Server
Would allow players to run the smoothest preforming RP for their client.

Possible Use Case:
http://www.reddit.com/r/Minecraft/comments/1ivbhn
That is a excellent example of content that would break miserably if this wasn't added, basically it is a resource pack that replaces Chinese characters with Images (via their Unicode map in a resource pack)

Challenges Faced:
None Discovered at this time.

Terms used
RP = resource pack