timppu: 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.
Just to chime in, haven't played the EE versions, but I surely noticed the CPU usage on my laptop. Someone made a mod if you have both BG1 and 2 to run BG1 on BG2's engine which is more CPU friendly.
Basically before I ran that I made a separate power profile on my laptop which would restrict my CPU to 30% of it's capacity, BG ran fine even that low, everything else would start to be slow as molasses if I forgot to switch power profile back. Basically the game just ran as it could without waiting for a set amount of ms to have passed (hey checking if the time has passed and wait uses resources we might not've had back then)
Now a days games expect there to be resources free. Whether it uses it depends. Also something like turning off VSync can cause the same behaviour in systems. It might speed up your experience with a low spec computer at the cost of screen tearing, but at the same time, it won't nescesarily throttle itself down to your monitor's framerate.
And then there's games which still don't care. It was the last straw which made me drop Transistor.