Based on TT's other Lego games, and guesstimating LCU will be on the high side (like Lego Batman 2) with all the time and care they've put into it, I think the biggest stopper for a lot of people may be a relatively relaxed, slow pace.
That's what the Lego games are. They have to be tuned so that both true kids (not just teenagers) and adults can play the games at both their levels and have fun. So LCU is probably not going to be a heart stopping game, and probably will not "blow you away" in the way that a lot of big boy games are designed explicitly to achieve.
Having said that, the strength of TT's better Lego games is in the granularity of the game world, where exploring and interacting with the environment is made fun. There's always stuff to break, studs (resources) to hoover up, secrets to find, and gadgets built into the game world to operate. The Lego games are not very difficult, they're not overly intense, but if you can get into them they keep your attention engaged. Because you're always doing something and usually with a lot of variety.
For this game, I'm hoping what helps carry it is the theme, characters, humor, and such. Since what we've seen in those areas is very promising. It looks like TT has been very much freed up to go all-out with their imagination by not having to work with a license. There may also be some concern about a large open world maintaining the packed nature of previous Lego games, so that it doesn't get boring between major points of interest. The fact that there's a lot going on around the buildings and on rooftops in the city is promising.
Essentially, before anyone gets unrealistic expectations, this isn't going to be GTAV. It has the potential to be a "soul food" game however. Pretty much just a celebration of the mechanics and activities at the center of playing video games.