Funny thing is SI was considered good because it tore down the generic feeling of Oblivion and was pretty much one of the few reasons to get it because of the change of bland dungeons, but once you got to it SI still had a lot of bland and boring dungeons and the amount of quests were smaller then Tribunal's. SI is probably the most overhyped DLC because everyone forgets everything else due to Sheogorath, one of the most beloved Daedra Lords in the TES series, in truth SI wasn't really that great.
Yeah, while I found SI to be a great expansion, the primary draws were the main storyline and a new landmass, while the side-quest content seemed pretty anemic, if memory serves.
I think Bethesda made a vital mistake with Dawnguard, they attempted two separate factions, but with little content to differentiate them. Sure, as a poster above mentioned, the dialog was different on either side and the radiant quests were different, the main story missions were nearly identical, at least so far on both of my playthroughs. If Bethesda wasn't planning on making very distinct separate questlines they should have focused their efforts on one splendid faction plot. And I'm willing to bet that many didn't take the time to complete both faction lines, could be wrong though.
Honestly, in my opinion 200-300 points should have been shaved off the price of this add-on. Dawnguard is obviously much more expansive that KoTN, but I do believe it is over-priced, at least somewhat. But, again, I found this to be an enjoyable DLC worth my time. I'm now wondering how large Hearthfire will be, I'm hoping it is at least on par with Dawnguard, but more reasonably priced. I would be ecstatic if Hearthfire turned out to be one of the more content heavy DLC.