The first thing you need is a trajectory which is close to orbital for worldwide reach, so check. The second thing you need is protection of the warhead on ree

The first thing you need is a trajectory which is close to orbital for worldwide reach, so check. The second thing you need is protection of the warhead on reentry. Kinda not check, the reentry profile of slowing down as it does makes it an easier target for counter measures. It could release a payload which is capable of reentry on its own, but Starship offers little advantage here. Third thing you need in a "practical" sense is rapid launching. If you're using it for a mutually assured destruction counter attack, it needs to be launched within minutes, as you only have 40 minutes from enemy launch before their nukes start hitting your launch sites, including any time for command staff to give the signal. A lot less with a submarine attack. Strong not check. Starship is huge so it has a long fueling time. Additionally it has a limited time that it can be fueled and be launch ready. You could maybe use it as a first strike, but you'd have to get a large number of them all fueling at the exact same time without anyone being curious why. Really, orbit to payload rockets and ICBM share a lot, but the specific design constraints mean there's important differences. It's payload capacity means it could possibly launch a large number of space based weapons, which is pretty scary. However there's little reason to use it as an ICBM specifically.