There is one big disadvantage I've seen to running in 1.7 over 1.6: 1.7 packs run near-universally lower FPS, and I have no damn idea why.
Everything else about 1.7 is far superior to 1.6, but for some reason my PC has to work a lot harder to do it.
Probably because OptiFine is broken with 1.7 versions of technic mods? I do not include it in any of packs, and where it is, it blocked something....so maybe that is the problem
@fire on sun : I have such similar mod, but reverted. It is called Toxic Rain, in intoxicates people on rain, so they are forced to have roof for sure
But I seen sometimes in past some mod making ppl burning, will try to find it
EDIT: Found it, but for Bukkit http://dev.bukkit.org/bukkit-plugins/sunburn/