The game-story unfortunately ends too quickly,
although there are such a huge amount of side quests left.

They should at least have implemented a "continue playing" options.

Basically I play the game, avoiding the last steps of the main quest, just to see the sidemissions.
This is not the best gamedesign.

They should have extended teh main story by including more of the many
sidequests as mandatory to continue.

i suppose they wanted to simply keep the "required" time low, to
finish the main plot.

Another thing:
the game is not well balanced when playing a long time.
After a while you get to be a super badass, killing everyone
without any difficulty.
Only some Deathclaws might pose a little thread then.

They should have included a part of the world as "superhard" area, where even hightrained characters have problems.

Gothic 1+2 solved this much better by using "Chapters"
where all creatures respawned / world got refilled, and got to be tougher new creatures.
(but not like in Oblivion, where they simply got harder to beat)