I've participated as a player in Sunless Citadel, and now I also own this module. There are a number of things that I dislike about it. (Warning, there are spoilers below!)
First of all, the encounters are not balanced for a typical 1st level party. For example, on the 1st level of the dungeon, the Water Mephit is too difficult for a 1st level party, and the Troll is simply way too difficult for even most 2nd level parties to deal with. It's actually not too difficult for the party to obtain the key which leads to the Troll, so don't assume that the magical lock is going to keep them away from this area. The question is, what are these doing here? The adventure clearly states in its introduction that it is suitable for four 1st level characters, but these kinds of encounters will tear them to shreds. Even the white dragon that they are more or less obligated to rescue given typical playstyle is likely to be quite difficult for low level characters.
As the party descends to the next level, they face even worse dangers. Balsag is a difficult match with his relatively high armor class, and then there's the shadow as well as the final confrontation with Belak and company. Considering that the party is unlikely to begin the adventure with any magical weapons, and there aren't many to be found in the adventure (some of the ones that are may be easily missed), the shadow seems totally out of place here. An experienced group may be smart enough to try running from it, but a group less experienced will invariably attempt to fight it with normal or silver weapons only to lose a party member or two before realizing their folly. Some DM's like this style of "learning" thru character loss. Personally, I find it pretty sadistic.
Even the low-level encounters are trumped up with terrain situations. For example, the initial encounter with 3 dire rats is made quite difficult by forcing the characters single file down a rope and then having to fight on a narrow ledge. It's very likely that the 1st character to climb down (likely a rogue) won't spot the rats that are hiding in the debris, and as they descend they will be exposed to 3 ambush attacks. Then depending on how initiative goes, they may well suffer 3 more attacks before they can respond. The fact that the rats can easily flank the lone character, and any help will take at least a round or two to arrive means that it's very likely this initial character could be killed by these rats. On top of that, it lists the falling damage from this ledge as 8d6. I don't recall ever seeing a "so-called" first level adventure that had any chance for anything dealing that much damage. I realize that it's unlikely that anyone should fall, but still this type of damage seems well out of place for an introductory level adventure.
Another trumped up area is the Goblin Gate with the caltrops. There's no way that an EL 1/2 can be justified for the hallway plus gate. The bell will likely sound even with an adept 1st level rogue, which will alert the goblin guards who will then pelt the party with javelins from 3/4 cover. There's no easy way to close quickly. And this gives time for reinforcements from the rooms behind to arrive. The +7 AC makes their effective Armor Class a 22, which is pretty much impossible for most 1st level characters to deal with. Such "trumping up" of encounters is generally a tactic one uses for higher level parties who face lower level foes. That's not supposed to be the situation here with 1st level characters.
Beyond these unbalanced encounters, I found the dungeon to be very large and unexciting. I actually like most dungeon-crawls, but this one just got monotonous. Considering that the 1st level is huge (41 rooms), it's likely going to take some groups several play sessions just to work thru that. It took our group 3 sessions to get thru most of the upper level. The lower level is about half that number of rooms, but it's even more difficult to handle for the reasons mentioned above. By the time we finished the upper levels, we weren't terribly excited about having "yet another level" to deal with.
It's very likely that a party delving into the Sunless Citadel will need to make numerous withdraws back to a safe location, perhaps as far back as the city of Oakhurst to rest up from their wounds, learn new spells, and make purchases. Personally, I dislike that tactic, and most of my players dislike it as well. Large dungeons have their place, but an introductory adventure is simply not the appropriate place for one.
I also found very little opportunity to role-play. A few exceptions are a couple of the Kobolds. Unfortunately, Meepo is likely to merely provide some comic relief or just be an aggravating NPC rather than provide any real role-playing opportunity for most groups. There's very little personality info listed to give Meepo any major direction. An experienced DM will likely be able to handle this, but a newer DM may find it difficult to run Meepo without turning him into a comedic or typecast humanoid. The Kobold leader is likewise left with very little detail regarding personality. There is the gnome, Erky Timbers, that can be rescued. He at least has a small amount of personality info listed, and is willing to join up with the party. But some groups may not want to take on an NPC due to the potential loss in Exp. Instead, it's likely they'll show him the way out, and leave him behind, along with any chance for roleplaying that he might bring. Beyond that, I found very little opportunity to role-play until near the very end of the module. The opponents on the first level consist of primarly of rats, undead and those goblins which are specifically listed with the note that "Diplomacy is not an option". The final encounter w/ Belak will invariable begin with some role-playing, but by this point, a smart party will likely want to cut to the chase and head directly into combat. (That or your play group might just decide to rush in to finally get this overly long adventure over with!) Considering that this dungeon has over 60 encounter areas, and only about 4 of them provide the party with any real role-playing potential, I'd have to say that the chance for role-playing in this adventure is pretty minor at best.
Also, the adventure is not very gripping. There's really not much that gets the group involved to explore further. The listed character hooks aren't terribly compelling. A weak party might decide to give up part way thru, especially if the encounters are simply too difficult for them to handle. And there's really little mystery behind the events going on here. A smart player will easily see the link between the apples, seeds, twig-blights and so forth, and then inform the Townsfolk of this quite early in the adventure, but the adventure doesn't really consider this approach.
Unfortunately, I'm left with a bad taste from this adventure. It reminds me of some of the junk that was getting pumped out of TSR during 2nd edition merely to make money. It feels like a long cumbersome adventure with little rhyme or reason in many of its encounters. Several encounters seem placed here merely to fill space rather than to make sense. In several places it seems like the author was simply trying to "show off", especially in those trumped up encounters as well as some of the traps.
It appears WotC is trying to produce adventures that bump characters 2 levels at a time, and I think this is a huge mistake. They should have produced twice as many adventures of about half the size. I'm actually surprised that they didn't do this since it would likely have earned them more income. I would have much preferred a simpler dungeon with fewer rooms, fewer encounters and a few more situations to role-play events. Many skills and feats will be largely unuseable in this adventure which will probably annoy your players as well. I expected a lot more from WotC for their first 3rd edition adventure, but in my opinion, they missed the mark here by a longshot. Other companies' (Necromancer Games, Green Ronin, etc) intro adventures put this one to shame. Hopefully, WotC will wake up and start putting out some better modules.
