The main problem with Tilke tracks is that they look too much alike to each other. That's what people mostly mean by "lack of character", it's the lack of any defining characteristics that you look at, and immediately associate with that particular circuit and no other, be it a particularly unique corner, unique scenery, a danger element, a relic of history. Truly great tracks have it.
But, there are other, many problems:
- Tilke tracks are just too wide. This was originally thought a good idea to promote overtaking and multiple lines, but greatly reduces the sense of speed on camera which makes it look unexciting.
- A recurrent pattern of a hairpin leading to a long straight leading to a hairpin. Again this was initially thought that it would promote overtaking, but empirical evidence over the last few years tells us otherwise, apart from a few exceptions. By forcefully introducing at least 1 section like this in every one of his tracks, not only he makes them lose uniqueness, it also constraints the layout as instead of this standard section you could've had far more interesting corners. And nowadays it's pointless to design tracks to induce overtaking as DRS takes care of that anyway.
- A billiard smooth track surface. This is now seen as the norm for every permanent F1 circuit but again makes it look unexciting on TV. Tracks with bumps and natural, not carefully designed, camber changes, just look far better and are more interesting from a drivers' point of view.
- Lack of flow. It's hard to pin down exactly what makes Tilke tracks flow poorly (you can check this out in driving games/simulators), as "flow" is always a subjective thing, but they are just not that enjoyable to drive, even COTA who is acclaimed as one of Tilke's best is clearly one of the most annoying circuits in my iRacing collection. I'd say it's a combination of having too many corners (almost every single Tilke circuit has more than 20 corners!!), too many of them being too slow, an unenjoyable exccess of short radius corners (where have long constant radius corners gone in modern circuit design?), too many corners designed "to force driver's mistakes" by having blind apexes or awkward approaches, the aforementioned lack of natural camber/elevation changes, and the massive runoffs that not only again contribute to the lack of sense of speed, but also make it more difficult to find references for your driving such as braking markers.
I think there's also been a lot of misconceptions in this thread:
- Overtaking opportunities are way overrated as a key factor to a circuit being great, I'd much rather have an enjoyable challenging layout than one that promotes overtaking but is sterile. Bahrain has lots of overtaking, and is a piece of crap. Whereas a onboard lap of Monaco by a powerful car at the limit will keep any true motorsport fan in awe. There's a difference between great circuits and circuits that promote great races.
- Corners that promote or punish driving mistakes aren't necessarily great. Tilke tracks are full of them, the awkward corner entries I've mentioned before. They are challenging but they aren't fun. Truly great corners are challenging AND fun, like Eau Rouge or Pouhon. These are corners that are fast, entertaining and not awkward.
I could write about circuit design all day but you get the general idea by now.