Not sure if anyone has brought this up yet or not, but I just played Nami a little while ago. I think her skills are awesome, but I feel her actual mana pool is a bit lacking for being a support. The cost of her heal was pretty well balanced to me, but even with buying mana regen and mana items, I was still constantly running out of mana. It actually made it quite difficult for me to support my team. I'm sure I could counter it with some runes and different masteries, but overall I felt her mana pool to be a little too small for level 1.
Sure Sona and Soraka barely have 300 mana if you go pure support with mana and mana regen, but Soraka also has a skill that has no cost and Sona has 7.0 mana regen (+.65 per level) to compensate for how often she spams her skills. Nami on the other hand has 6.9 mana regen (+.6 per level), and her cooldowns are low enough that she could potentially spam her skills, especially her q.
I'm sure this won't be changed since the 2 main supports don't have as much mana as Nami, but I felt that for level 1 to 3 she was mana hungry to the point that it was getting hard to compensate with items. I also know there are better supports to compare her to, but those were the one's I chose to go with. Another good compare would be Janna, who starts with about the same amount of mana, and goes through the same problem from level 1 to 3.
Perhaps it isn't even the champion mana pool that needs fixing, but supports in general. Most support champions are mages, and some adc's have more mana than their support, who is supposed to be spamming skills to keep them buffed/alive. That's just how I felt, not sure if anyone else has felt this way, and I'm not sure if the developers feel that way, just wanted to throw this out there for them to see.