Wow, it's amazing that there are such varied demands from an online tabletop RPG aid. I feel bad for those folks who are designing this stuff... their customers want an incredibly wide variety of functionality and features.
I like the visual dice, but I've rolled enough dice in my life that it's not a big deal. I tend to like more automation of basic things like attack rolls, saves, skill checks, damage, etc. If that stuff is automated, combat goes *ever*-so-much faster, and we can accomplish a lot more than a simple skirmish in 4 hours of playing.
I love the map features, and yeah, zooming is a must. Cool tokens and figs also add a lot to the map's usefulness for me.
The chat interfaces I've seen are hopelessly inadequate for my needs. I put a great deal into my NPCs, and "you can talk as them" is nowhere near enough. I need to have multiple NPCs that I can control completely and easily. I need them to have persistent descriptions, hit points, skills, etc. I need them to be able to interact with each other and with the PCs, as well as the environment (weapons, armor, etc.) That's a level of functionality I've not seen in anything other than a MUD (which is why I went with a MUD to run my games on). This is my #1 need... communication interface.
Closely linked to that is multiple rooms, described and interactive, with exits and entrances, etc. A picture might be worth a thousand words, but a picture can't describe things as well as a single paragraph of text can.
Environmental controls (lighting, silence, adjacent visibility, etc.) and targeting are additional perks. None of the bare-bones chat interfaces I've seen offer anything remotely close to these features, which kind of surprises me. I think that my needs are probably different from those of many users.
Sound is nice. That's one of FG's big drawbacks for me... I have to host my own sounds and provide URLs for my Players to hear them. That's a major pain. FG allows me to present additional graphics as a "map" which works out wonderfully. From NPC portraits to various graphics, I can do it in the same application as the map, and that's sweet. I don't use any animated graphics (other than some simple morphing GIFs) so multimedia support isn't useful for me.
Stability is important. I've left FG running for 24+ hours, and people can connect and disconnect without problems. That's awesome. I've not testing FG with 5+ users, but it's perfectly speedy with 4 users (including the host). That's awesome.
Getting FG to punch a hole through the firewall/IP was a major pain in the butt, and there were very inadequate instructions to accomplish it. Luckily, one of my Players had already done it, and he walked me through it. Otherwise I would've thrown in the towel on FG. I also don't understand why the software can't configure access itself, rather than requiring a LAN admin-comparable level of expertise. If you want to target only the highly-technical-savvy users, make 'em configure their internet access themselves. If you want to capture all of the non-techie users as well, you have to do it for them. Other programs do it, and I suspect that FG and Battlegrounds *could* although it's probably a programming nightmare.
Anyway, those are my thoughts on the various platforms after playing 4 sessions so far with FG and Hesperian.