Floating bodies of land in the air:
I am almost certain that Delgo isn't the first to conceptualize this. It's something that can be found in several science fiction or fantasy stories.
The Na'vi:
I found little to no similarities with the image from Delgo. The skin is blue in opposed to tan. The hair is long a tribal, black instead of brown. The clothing is more primitive. The noses are extended further down, with the lips closer to the nostrils than in Delgo. There is also the lack of facial hair, in any of the Na'vi that I've seen, as well as the Na'vi having pointed, elf-like ears, while Delgo has rounded human-like ears.
Hostile alien:
The one in Avatar appears more amphibious than insectiod, having no visible carapace. The one in Delgo has a clear shell on it's back, while the one in Avatar has what look like flower buds. Avatar has legs that appear to have bones in them, while Delgo has crab-like legs. The Avatar creature has what appear to be only two eyes, while the Delgo creature has several. Unsimmilar, if you ask me.
Interaction with glowing spore:
Another common occurrence. Be it fairies, phosphorous flowers, or other some sort of unearthly phenomena. This is even seen in Ferngully. Even then, the spores seem different, and less of a romantic occurrence in Avatar.
Flight on winged creatures:
When I watched the Avatar trailer, the first thing I though of was the Skybak Riders in Dinotopia. Even flight on the back of Dragons has been done before. The Creatures in Avatar have webbed wings, more resembling dragons, while the creatures in Delgo seem to have a single membrane, much like a Quetzocoatl. While similar, they are different.
Death of a loved one/warrior:
A concept as old as time. We see this in 10,000 BC, Lord of the Rings: The Two Towers, etc, etc. Saying that Avatar copied this from Delgo is like saying Saving Private Ryan copied WWII battle from Patton.
Romance between major characters:
See above.
Dragon-like creature:
Avatar's has pointed teeth, while the Delgo creature has what look like flat teeth. There is also skin color difference, vestigial horn differneces, and the eyes. Avatar has two, while it appears Delgo's has four.
Warriors ready for battle:
Avatar's once again seem more primitive and tribal, while Delgo appears more like the Elves from The Two Towers. Delgo seems more organized, with structured buildings in the background. Avatar has wooden weapons and leather armor, with tribal huts. I fail to see the similarities.
Overall, I would say that it's possible that Delgo inspired Avatar in ways, but the two are not a blatant copy+paste+edit. They're as similar as Halo and Starcraft are.
Current Fan Fictions in the Library:ODST: Hellbound
The Interrogation of Ellen Anders
Halo: Archangel