You can enchant both helmets and circlets... It's just a balance issue. Too many pieces of clothing/jewelry with the easy and cheap enchanting system in this game would make you a near God just minutes into the game.
IMO yes there is the balance issue. But people that aren't abusing the exploits don't really get much in enchanting stuff. Morrowind and Oblivion both had the second ring and I was NOT over powered. The problem with Skyrim is they tried a new area of the game mechanics like crafting and didn't do very well. I saw this coming before the release date. BGS and other single player game companies do not know how to balance crafting professions like the makers of MMOs. Example in AION.... When you crafted to a certain level. The game forced you to make more complex and higher quality items to gain XP to the next level of crafting. Which IMHO kept things balanced. BGS didn't take this into account and thus we now have major exploits. Also I doubt I could be Godlike just minutes into the game with enchanting. I do not think +5 fire dmg or +10 Resist or HP is going to get you very far. When you start the game, you can buy better enchanted gear then you can make by far.