I don't think we should be saying 'porting' all the time. People say they want stuff like MMM and FWE to be 'ported', but that's not possible. FNVEdit allows you to 'port' some smaller mods (like simple game setting tweak which is identical in NV), but most of the time I think mods have to be re-made to fit New Vegas, e.g. any mods that actually add something to the game world.
I don't like the word "port" either, "migration" is much more accurate.
1. The NV-GECK makings Migrating mods from Fo3 to FNV quite simple; it smacks references it doesn't know about anymore, and after switching master files and loading it into the NV-GECK, what your left with is entirely legal (assuming no Fo3 assets were added outside of the normal form IDs). This makes Most mods viable for migration to New Vegas at least in skeletal form. What gets transferred however depends on what the mod was made of.
2. I would argue that Most mods can be migrated, including ALL quest mods and adventure / new levels/world mods. I know this from experimentation with other mods, my own big mod and in writing a complete tutorial on how to migrate mods successfully. It is entirely viable and also requires Some work on the mod maker depending on how lucky we got. By that I mean how many common-assets we each used that are in both games.
3. The core process of migrating a mod from Fo3 to FNV is very simple, where the process of "fixing" and "tweaking" the mod before and/or after migration can be much longer (again) depending on what assets you used. I think in my case the worst cells were my Statesman Hotel (3 floors). When I loaded it, 90% of all my clutter items were common to both games and came on through - but the room pieces themselves did not (walls, floors, ceiling, etc). Looked humorous to say the least with all that clutter just Floating in the air.

However this only took 5 days to fix; taking hotel pieces in New Vegas and using a view of the Fo3 GECK on my other monitor as a template to re-build the missing room pieces from New Vegas assets in the NV-GECK. Result: 5 days of work to successfully migrate 3 cells that took six weeks to build originally. This was my worst case.
The legality is 100% focused on the assets; you can't use models, textures or sounds in New Vegas mods unless those assets exist in Both games. The tutorial we wrote (FNVEdit in sig) also reviews a process of replacing Fo3-only assets with common assets; something JustinOther helped develop and correct. I used it on my quest/adventure mod and only lost ~2,200 assets out of ~43,500 total assets - I considered myself very lucky and it will only take about 4 weeks to complete the retrofit to New Vegas format (after which I can get back to New content).
To conclude, it is 100% viable to migrate mods to New Vegas, and 99.99% likely that some of the assets we used in our mods Won't make the migration and will have to be replaced with New Vegas-only or common to both games-assets. For some mods like FWE which changed all the weapons around, I would say these are the Least likely to migrate as almost nothing would come over right, the guns wouldn't work, etc. Others who added just adventure/quest mods, there could be very little to change after migration.
Miax