I think the main problem of AoS is that almost all buildings are useless. Bridges, towers and bunkers just makes you an easy predictable target. It also motivate griefers. Tunnels and towers for intel are only really useful things for current gameplay. Am I wrong? If not, what to change?

I disagree,
Towers/bunkers etc. are very good for protecting bases and intel.
Yes, they're basically useless if you set one up in the corner of the map, or if you're building them as an offensive player.
Having them set up in strategical positions, however, can really change the way the battle is fought, and give you the upper hand in certain situations.

I agree with Kanye, built in the right place a fort can be awesome.

usually where there is a choke point and the enemy are advancing, a fort staffed by a few people can really make a difference. I was playing the other day and found myself building a fort that was borrowing heavily from medieval castle design - e.g. a main reinforced doorway, moat for grenades to fall down into instead of just blowing away the foot of the wall, tower covering the main doorway (i.e. sideways looking etc). Great fun to defend from, and great fun to attack.

Buildings are only useless and only make you a sitting duck when you end up building a lame "snipers nest" - e.g. a tower with a single platform/window for people to camp in and try and get easy kills. I dislike the "L33T SNIPER" mentality immensely so am happy with this :-)

When you are advancing and you see a structure looming through the fog and its got lots of windows or doors or other places that people could shoot you from, that really makes you think as someone could pop up in any one of those windows and shoot you and you can't cover all of them at once so you have to change your tactics instead of just running hell for leather straight into their base etc.

Exactly^

Another example, I was in a game just now (bridge wars, but all the bridges were griefed) and I started building a wall, with a few shooting spots out of it. After a few minutes at least 3 or 4 people started helping me make it, and it turned into an almost three floor giant stronghold, with pillars for support (supports make it harder to grief), and needless to say, there were like, no blues making it past us. It was excellent.