Fairfox: Gosh. I have the originals (well, of the B.G. titles) but never played them, and have been put off by the thought of 'modding' (shudder!) to get them to a reasonable level.
I played the original Baldur's Gate 1-2 and Icewind Dale (GOG versions) vanilla, no issue there. So it isn't a requirement to mod them or anything, you can also enjoy them vanilla (I usually like to play my games vanilla, at least the first time, unless there are some game-breaking bugs). The only thing I didn't like was the journal (quest log) on BG1 and IWD, the BG2 quest log is better (easier to tell which quests you've already finished and what to do next).
As already pointed out, the enhanced editions also use your CPU power in a saner way. This might be mainly an issue for laptop gamers where heat problems may occur (or alternatively you'll hear your laptop fans roar at full speed all the time) if the game tries to use CPU core at 100% all the time (on newer multicore laptops it is normally 50% though, due to HyperThreading I presume). And, if this becomes a problem (as it occasionally did to me on a T400 laptop), the workaround is to select a power profile in Windows where CPU usage is restricted to low, when playing the game. Then the game can't steal excessive amount of CPU cycles for no good reason.
http://www.gog.com/forum/baldurs_gate_series/baldurs_gate_12_enhanced_editions_also_100_or_50_cpu_usage/page1 Specifically for BG: EE, its video parts (like the intro etc.) are poorer than in the original. I presume this is because the game is designed also for mobile devices where there is less space for videos, so they replaced them with 2D animations or slideshows, and apparently some videos are even missing from the BG: EE.
BG2: EE doesn't seem to have the same issue, I think its videos look just as good as in the original PC version. Not sure about IWD: EE.