The island of Solstheim is fairly large. It contains three hub towns where you can meet people and pick up quests, and there are over 40 other map locations, including dwemer ruins, bandit camps, and creature caves, where you can complete quests or just explore for the sake of exploring. I spent 20 hours playing Dragonborn, and I didn't see everything, so the DLC pack has a nice amount of content for its $20 price tag.
The main questline for Dragonborn consists of seven quests. When you arrive on Solstheim, you discover that the people there are a little bit muddled. They're building shrines to Miraak but they don't know why, and they've heard of Miraak but they don't know from where. So that means you first have to free the people from Miraak's control, and then you have to seek out the power necessary so you can defeat him. Amusingly, the latter part involves tracking down some Words of Power and riding a dragon, which should sound familiar to anybody who has completed Skyrim's main questline.
The best part of Dragonborn is the black books. While you're searching for information about Miraak, you stumble upon some black books, each of which transports you to Apocrypha, the realm of the daedric prince Hermaeus Mora. Apocrypha is sort of like a large, evil library, complete with writhing tentacles, moving passages, tongue-like ramps, and of course a huge supply of books. The visual style for this realm is completely different than anything else in Skyrim, which is great, and Bethesda showed more restraint with it than usual. You're only allowed into Apocrypha a handful of times, and you run out of reasons to go there long before the realm wears out its welcome. You also get a bonus for each black book you read, and these bonuses include useful things like removing friendly fire against followers and being able to summon a merchant so you can sell your excess items.
Other additions in Dragonborn include gem geodes, which you can mine for gems; craftable armor sets, including bonemold and nordic carved; shouts, including Dragon Aspect, which gives you a significant bonus to weapon damage and shout power once per day; enemies, including rieklings (pygmy scavengers), netches (floating jellyfish-like creatures), and albino spiders; crafting materials, including stalhrim ("enchanted ice") and netch leather; and more. Unfortunately, as far as I can tell, these additions are restricted to Solstheim, so they won't change anything in the main part of the game.
One of the new shouts in the DLC pack also gives you the ability to ride dragons, but this feature isn't implemented especially well. You're only given minimal control over a dragon, so you can't use it like a traditional mount, and combat while mounted isn't fun at all, as dragons still take their time circling and sweeping when attacking a target, and so battles take forever. That being said, you can use spells and shouts while mounted on a dragon (but not bows for some reason), and so dragon riding might be a better fit for spellcasters rather than the melee specialist I was playing.
Sort of oddly, Dragonborn is mostly aimed at mid-level players. I started the DLC using an old save from my first character, and then I took him from level 30 to 38 while exploring Solstheim. Some of the battles were tough for him (mostly because his magic resistance was minimal), and he had to keep his weapon charged, but he died almost not at all, and the final battle against Miraak was a yawner. If you have a character who can defeat a dragon priest, then you should be able to play the DLC without any problem.
The exception to the above -- and the reason I included the word "mostly" in the first sentence -- is a pair of level 80+ enemies. One of these enemies, the Ebony Warrior, can only be encountered if your character is at least level 80; the other, a frost giant named Karstaag, can be encountered at any level if you summon him. My character tried out Karstaag, but let's just say that the battle didn't go very well for him (he ended up stunned the entire time). So if you have a collection of high level characters, then Dragonborn might have some challenges for you, but I'm still surprised that Bethesda hasn't created a high-level DLC. I'm sure most people with Skyrim have some level 50+ characters sitting around with nothing to do, and the three DLC packs released thus far haven't really addressed them (and even the high-level enemies in Dragonborn are sort of odd; it seems like it would be a chore and a half to grind a character all the way to level 80).
Overall, Dragonborn is a nice DLC. It contains numerous additions to the game, some large and some small, and while they don't all work perfectly, Bethesda at least gets an "A" for effort. In particular I enjoyed the black books and exploring Apocrypha, and these sequences make up for some of the more mundane parts of the DLC, including the side quests (the first three side quests I picked up were all fetch quests, which didn't exactly generate a lot of enthusiasm). In the future I'd like to see Bethesda create a DLC for level 50 characters, but until then Dragonborn gives you a lot of new stuff to do, and it seems like a worthwhile purchase at $20.