Y'know, I have never heard of Ravenloft having a bad reputation. Not even once.
And it doesn't take a "very good" DM to play it right. It just takes one who reads the actual adventure and isn't a jerk, which are both bare minimums for DMs.
From 1d4Chan:
What is Ravenloft like?
The setting has been described (by Noah Antwiler) as "Hell, but not for you".
The first rule of Ravenloft is not to touch anything, ever. Assume everything is cursed unless you saw someone pick it up and put it down without becoming a zombie, and even then that's no guarantee. Second, alignment-detecting magic can only say if something is lawful or chaotic, so you can't use those spells to check if anything is
safe. Third, beware of curses, as they're especially powerful and anyone can place them if they're angry enough and get the Dark Powers' notice when they speak the curse. Any deed that could be considered "evil," like unprovoked assault, murder (especially of family), oath-breaking, or using specific naughty spells (usually necromantic) pings the Dark Powers' attention and calls for a "Powers check," a percentile roll against a number determined by the act committed and the victim. Roll above the number and you're safe (for now). Roll below and the Dark Powers like what they see, "gifting" you with something that seems beneficial, but pushes you to commit more nefarious deeds, which prompt further Powers checks. Soon, the Dark Powers' gifts come with obvious curses and debilitating drawbacks that end in either your death, transformation into something inhuman, or "ascension" to the rank of Darklord.
Also, you can't leave unless the Dark Powers let you. There are rumors of other ways out, but they are always unclear and extremely dangerous to attempt. Attempting to use
plane shift or other dimensional magic never get you out of Ravenloft; each domain is treated like its own plane, so you'll likely end up in a different domain instead. 3.5 allows to get in and out
via OP1's World Serpent Inn, which shows up in the Demiplane of Dread at certain set intervals. The WSI fistula pisses the Dark Powers off to no end to the point they immediately threaten and scare anyone away from the door that leads to the inn as and when it appears in their domains. This whole "You can't get out unless we let you" schtick gave birth to so called "Weekend in Hell" adventures, where the players act like the unwitting pawns of the Dark Powers to torment a Darklord and are magnanimously granted a ticket out if they succeed without being corrupted themselves.
At the hands of a bad DM, 3.5's security-breach allows spells and powers unique to the plane to seep elsewhere, like into the
Forgotten Realms. One of the prime reasons that make the Demiplane of Dread so dangerous is that it's home to spells so broken and dangerous that if they became planes-wide knowledge, everything could get screwed in a mere matter of days. A good example of this would be Strahd's unkillable zombies in the hands of a Necromancer with the right feats, resulting in zombies that never die, endlessly heal if limbs are severed, only to turn into more undead that also explode and heal other undead in an endless cycle. But the seepage problem wasn't a new one; it's long been known to
any worldbuilder, since Pizarro first stepped into Peru. DMs with three neurons to rub together can rule that the spell or ability in question simply ceases to function anywhere outside its domain of origin - so restraining Strahd's super-groovy zombies, in our example, to Strahd's prison.
Incidentally, as a result of the "no one can leave" thing, using conjuration magic is an
extremely bad idea, as most summoned entities will be quite upset when they realize they can't go back home when the spell expires - and they will usually take their anger out on the conjurer with lethal results.