Just a response to those above complaining about the lack of variety, and/or the choices made for the contents of the Core Box;
This is exactly how Descent started off;
5 Goblin Archers
5 Cave Spiders
4 Barghest Hounds
4 Flesh Moulders
2 Shadow Dragons
Based on the list above, there is only 1 more "monster type" here than there will be in Imperial Assault. Even with all these monsters, you can't use whatever you want in any given mission in Descent; the scenario restricts you to different classifications of enemies you can add in.
Future expansions provide more monsters, and these monsters fill in the ranks of the monster types, so you have more variety not just in the overall monster pool, but with each monster type to add into the specific scenarios. This is where the long term replay value comes in.
I can't imagine Imperial Assault not following this same pattern of small box/big box releases for expansions, and we already know they will have their own version of the Lieutenant Packs.
I know the Imperial Assault Core Set seems a little thin, but have faith, FFG will no doubt have already announced one or more expansions by the time Imperial Assault lands in stores.