I can explain by examples.
With 了：it can express complaint or compliment.
1.The house stinks so much! 房子太味儿了（太臭了）！
It's obviously a complaint.
2. Her voice is so lovely~ 她的声音太好听了～
And it's an obvious compliment.
3. The house is so/too big. 房子太大了！
big is a neutral word. How can we know it express what?
In this case, it express complaint, negative comment. Here it means excessive.
If you want a compliment,we say 房子真大!
Same as 太小了，太高了 《— negative
without了：I think in most cases it also express complaint. I often hear it from adults. but they are mainly the same. I think it's the tone.if you are a Chinese, you don't add 了,we can also read your complaint by your tone. But you better add 了 to emphAsize if you want to make sure them know you are not okay with something.
（maybe it's an indirect way of complaint? To be polite? It is just my guess.)