[Phanx]

Here's the code I use for a simplified text-only difficulty display in my private minimap addon:

MiniMapInstanceDifficulty:SetParent(Minimap)
MiniMapInstanceDifficulty:ClearAllPoints()
MiniMapInstanceDifficulty:SetPoint("TOPRIGHT", -2, 20)

MiniMapInstanceDifficultyText:ClearAllPoints()
MiniMapInstanceDifficultyText:SetPoint("RIGHT", 0, -9)
MiniMapInstanceDifficultyText:SetFontObject(TextStatusBarText)

MiniMapInstanceDifficultyTexture:Hide()

GuildInstanceDifficulty:Hide()

MiniMapInstanceDifficulty:HookScript("OnEvent", function(self, event, isGuildGroup)
	if event == "GUILD_PARTY_STATE_UPDATED" then
		self.__isGuildGroup = isGuildGroup
	end
end)

function MiniMapInstanceDifficulty_Update()
	local instanceName, instanceType, difficulty, _, maxPlayers, playerDifficulty, isDynamicInstance = GetInstanceInfo()
	local _, _, isHeroic, isChallengeMode = GetDifficultyInfo(difficulty)

	if MiniMapInstanceDifficulty.__isGuildGroup then
		instanceType = "GUILD"
	end

	local color = ChatTypeInfo[strupper(instanceType)]

	if color and maxPlayers > 0 then
		MiniMapInstanceDifficultyText:SetFormattedText("%d%s", maxPlayers, isChallengeMode and "++" or isHeroic and "+" or "")
		MiniMapInstanceDifficultyText:SetTextColor(color.r, color.g, color.b)
		MiniMapInstanceDifficulty:Show()
	else
		MiniMapInstanceDifficulty:Hide()
	end
end

local difficultyText = {
	DUNGEON_DIFFICULTY1,
	DUNGEON_DIFFICULTY2,
	RAID_DIFFICULTY1,
	RAID_DIFFICULTY2,
	RAID_DIFFICULTY3,
	RAID_DIFFICULTY4,
	UNKNOWN,
	CHALLENGE_MODE,
}

MiniMapInstanceDifficulty:EnableMouse(true)
MiniMapInstanceDifficulty:SetScript("OnEnter", function(self)
	local instanceName, instanceType, difficulty, difficultyText, maxPlayers, playerDifficulty, isDynamicInstance = GetInstanceInfo()

	GameTooltip:SetOwner(self, "ANCHOR_NONE")
	GameTooltip:SetPoint("TOPRIGHT", self, "LEFT")

	GameTooltip:AddLine(instanceName, 1, 0.82, 0)
	GameTooltip:AddLine(difficultyText, 1, 1, 1)
	GameTooltip:AddLine(difficultyText[difficulty] or UNKNOWN, 1, 1, 1)

	if self.__isGuildGroup then
		GameTooltip:AddLine(GUILD, 1, 1, 1)
	end

	GameTooltip:Show()
end)
MiniMapInstanceDifficulty:SetScript("OnLeave", GameTooltip_Hide)

Drycoded Broker conversion:

local DEFAULT_TEXT = ""

local isGuildGroup

local b = LibStub("LibDataBroker-1.1"):NewDataObject("InstanceDifficulty", {
	type = "data source",
	text = DEFAULT_TEXT,
	OnTooltipShow = function(GameTooltip)
		local instanceName, instanceType, difficultyID, difficultyName, maxPlayers, playerDifficulty, isDynamicInstance = GetInstanceInfo()
		local difficultyText = GetDifficultyInfo(difficultyID)

		GameTooltip:AddLine(instanceName, 1, 0.82, 0)
		GameTooltip:AddLine(difficultyText, 1, 1, 1)

		if isGuildGroup then
			GameTooltip:AddLine(GUILD, 1, 1, 1)
		end

		GameTooltip:Show()
	end,
})

local f = CreateFrame("Frame")
f:RegisterEvent("GROUP_ROSTER_UPDATE")
f:RegisterEvent("GUILD_PARTY_STATE_UPDATED")
f:RegisterEvent("PARTY_MEMBER_DISABLE")
f:RegisterEvent("PARTY_MEMBER_ENABLE")
f:RegisterEvent("PLAYER_DIFFICULTY_CHANGED")
f:RegisterEvent("PLAYER_GUILD_UPDATE")
f:RegisterEvent("UPDATE_INSTANCE_INFO")
f:SetScript("OnEvent", function(self, event, ...)
	if event == "GUILD_PARTY_STATE_UPDATED" then
		isGuildGroup = ...
	elseif event ~= "UPDATE_INSTANCE_INFO" then
		RequestGuildPartyState()
	end

	local instanceName, instanceType, difficulty, _, maxPlayers, playerDifficulty, isDynamicInstance = GetInstanceInfo()
	local _, _, isHeroic, isChallengeMode = GetDifficultyInfo(difficulty)

	if isGuildGroup then
		instanceType = "GUILD"
	end

	local color = ChatTypeInfo[strupper(instanceType)]

	if color and maxPlayers > 0 then 
		b.text = format("|cff%02x%02x%02x%s%d", color.r * 255, color.g * 255, color.b * 255, isChallengeMode and "++" or isHeroic and "+" or "", maxPlayers)
	else
		b.text = DEFAULT_TEXT
	end
end)