Vanilla Heretic doesn't have the fast door actions introduced in Doom 2 (because its code is derived from Doom 1), but I believe none of the vanilla Doom engine games (except maybe Strife?) have locked switches. I'm having a hard time finding more specific information, but I think locked switches were made possible in Boom through the use of voodoo dolls, and in ZDoom formats through the use of scripting. In vanilla Heretic format you can use the voodoo dolls, and in ZDoom's Heretic in Hexen format you can use the scripting. The latter should also allow you to use fast doors, or at least something functionally identical. On a related note, vanilla Heretic may have fewer linedef actions than Doom 2, but it also has more sector types, including the scrolling/pushing floors that allow for a huge amount of Boom-like pseudo-scripting.
Heretic's relative lack of textures does suck, though it isn't a problem unless you really want to use vanilla Heretic, which I would agree is not very viable for modding. Gameplay-wise, I think the biggest problems with Heretic are the weakness of many of the weapons and the fact that most of the monsters are very similar to each other.
I've been working on a content expansion for Heretic (monsters, artifacts, etc.), as well as a moderate rebalance of the gameplay. Most of what I currently have done can be found in the latest beta of The Wayfarer, so if you want to check it out and let me know what you think, the feedback would be appreciated. Also, if you want to see a more extensive list of all the little things I think are wrong with Heretic, you can check out the list of stuff I've fixed, which is in the OP of that thread.
@Xaser has a partial sound mod for Heretic floating around somewhere, which I've played around with and borrowed many fixes from (more recently than the beta in the thread linked above, unfortunately). The big takeaway I got from it is that Heretic's sounds are mostly fine, but there's a weird issue with the sound rolloff where all sounds start to become quiet immediately after leaving their source, rather than a few hundred in-game feet away from the source like they do in Doom. You'd be amazed how much better the game sounds just with the rolloff fix.