Let me just say this: Personally, I think Yas Marina, Marina Bay, and Korea International are actually pretty decent tracks for a driver, but they aren't so great to watch.
Tilke seems to have a love affair of hairpins after long straights. It's not a terrible thing, by any means, but it can get boring after a while. And it's almost like his only solution for creating overtaking opportunities.
What you don't see very often from him are chicanes. There isn't a single one in Korea, China, Bahrain, the current Hockenheimring, and the one in Singapore can be taken in basically a straight line. Spa has one before the start, Monza has three, and Canada has the infamous 'Wall of Champions.'
Which brings me to my next point, actually. Of the previous tracks I mentioned, none of them are really a high speed, low downforce track like Monza or Canada. Maybe I'm imagining it but most of his tracks are medium speed with a couple of high speed straights. If you recall, Canada provided a very exciting race due to the nature of the circuit.
Speaking of nature, some of the best tracks have a really organic feel to them. Spa is such a fantastic track because it's built into the hills where the natural geography shapes the circuit. Even without the rain factor, Spa would still be a fun race to watch. I remember when we all saw helicopter shots of the track, one poster said "Why couldn't they build the track in those hills over there?" and I certainly agreed. There aren't many elevation changes in Tilkes tracks. This point I'm willing to concede since the FIA is kinda strict on circuit regulations, which includes elevation.
Another complaint is that Tilke designs sectors that look like kart tracks. If it's a street circuit, sure, 90 degree turns are a given mostly. But then you look at the second sector in Bahrain (redesigned, mind you) and the last sector of Yas Marina and you wonder why he couldn't pull an Interlagos and make nice flowing curves. There's certainly room for it on the site.
That's all I can think of for now.