I tried to look for it around the forum, but could not find the answer.

People say like want to play imperials for theyre bonuses, and that they like the bonuses. But what are the bonuses? Do they just mean the mount and that?

They are not yet revealed by ZOS. So nobody knows. Guess not even the closed-beta participants.

Every race has a skill tree, just like every type of armor does. So different races have different abilities (similar to the single player TES games). Nobody knows yet what these skills are for the Imperial, though (and in fact the rest of the racials haven't been officially announced either).

The unique passive and ultimate ability has not been unveiled but will likely not be unbalanced in any way.

However they will be a unique in-game mechanic and likely impact how strong specific builds are with that race.

Something tells me it'll involve something like "+ % better prices from vendors", or something else to relate to the Imperial's more cosmopolitan and mercantile background.

All the other racials are combat related so it's a guarantee that these will be too.