What we earn and what we lost with this migration?
What should I expect as drawbacks after the migration?
Is it really unnecessary to change the applications in any situation?
I think now is the time to play with MariaDB (just like now is the time to play with anything like this). Personally, I would hold off until I know a few more devs who have deployed MariaDB onto a live customer site before doing it myself.
Some situations could call for a change of application but something as serious as changing your database engine is not something that should be considered lightly.
There is no reason to speculate whether Oracle "will" continue developing MySQL actively. The facts are easy to find out: look at the changelog and preview release announcements. The truth is that Oracle has accelerated MySQL development faster than it's ever happened before, and the releases are really good quality too, unlike Sun's 5.1 release or MySQL AB's 5.0 release, both of which were terrible quality.
Oracle deserves recognition for saving MySQL, not for abandoning it.
MariaDB can be just be plugged-in due to compatibility with MySQL. In addition, there are some good features for DB Administration (i.e. microsecond accuracy of query time instead of only seconds).
I have done that in several Drupal site and have had no problems at all.
Most my servers and clients have started switching over to MariaDB 5.2 as they still have predominately MyISAM based databases and from http://vbtechsupport.com/606/ it's clear that MariaDB is the one to choose if you want any MyISAM related performance gains.
MariaDB 5.3 is shaping up to very well too with mysql replication improvements and group commit see results at facebook's MySQL page as well as https://lists.launchpad.net/maria-developers/msg04278.html
I learned something interesting...
Baron Schwartz seems very confident that MySQL will stay strong for a while. Personally, I would not jump off the MySQL train just yet, either. Since MySQL (eh, Oracle [that just doesn't roll off the tongue, and it never will]) continues to be supported, this would give others such as Monty and Percona a chance to keep pace with MySQL's changes to MyISAM (though this might be rare). In the event ot such changes, Monty and Percona could implement corresponding changes to code for MyISAM and MariaDB.
You can think of MySQL a lot like Social Security in the USA. While there are many people speaking of Social Security eventually going away, getting out of Social Security now could rob one of the future higher monthly payouts/benefits. Drizzle went that route and froze MyISAM in time and utilizes that storage engine for temp table use only. Most seasoned Drizzle users are content with that decision, but this leaves no room for higher expectations.
So, until MySQL actually goes away, there is no real reason to abandon MySQL. Of course, by the time MariaDB becomes more mature and widespread, switching storage engines and DB servers is simply rendered academic and boils down to personal choice.