TH loot penalty should be made known within the game
How many of you upgraded your TH, not knowing that there was a loot penalty?
I surely did, back in December I upgraded to TH9 under the advise of another clueless clan member, because he said the X-Bow was worth it. My defenses were only TH7 level at best, with mostly pink walls. Here I am, over 5 months and still at TH9. It was quite a struggle to farm at that time, and I still have another 3 months+ to go before TH10.
If the TH loot penalty was made known to me from within the game (TH info screen? a 1 line message at the game load screen?), then I definitely would have looked into it first.
A very very small percentage of the CoC players actually visit the forum, and even if you do you must be lucky enough to stumble across a post about it.