Sure enough, immersion is one of THE yardsticks for *any* good sim imo. Unfortunatley, immersion is what most of the recent sims are lacking in. This is even true for BoF to some extent, as there isn't any real character development for the player, no crew/platoon members you need to take care of and of course no dynamic campaign.
Recent sims tends to focus on the technical aspect, that is, simulating aircraft, tanks etc., but it doesn't really give the player the feeling that he/she is involved in a military conflict where he/she is only part of something bigger. In the end, those sims are feeling rather dry and hollow.
I would love to see a truly dynamic tank sim, be it WW2 (my favorite) or a Nato versus Warsaw pact scenario for example, with news flashes to keep the player up to date regarding the overall picture of the campaign. And reinforcements should also be modelled realistically: for instance if you loose too many tanks on some missions, your company later might have a tough time to do the job on hand.