What do players expect out of medieval RPG servers?
Players expect to be immediately thrust into a immersive world, with quality builds, and lots of unique content to play. They expect up-most professionalism with everything, including the staff. They expect unique, and custom-coded plugins, not just for them to have different content, but for them to have something out of the ordinary to learn. I'd suggest not building a server around the use of plugins such as essentials and group manager, with custom plugins for quests, and so on, but have everything made to fit your server. Keep in mind, this may cost money, but in the long run, you will earn it back through donations.
What are some good plugins I can use?
Well, this completely contradicts my previous point, about using custom plugins, but you could use the following plugins to try and fit each category of an MMORPG experience.
Quests + Citizens
You can use citizens to have as NPCs, often an important part of an MMORPG, and you can have the NPCs made using Citizens to be quest-givers, using the quests plugin.
Mythic Mobs
The plugin Mythic Mobs will help in enabling you to create immersive, and interesting dungeons, with custom drops for your player to earn, through playing.
For those of you that use Spigot: Is it easily manageable? Spigot is basically the same as Bukkit, from what I could gather through all my usage. The only major difference is the proclaimed performance enhancement, however, I don't really feel it's too big of a deal, as it's the server host that will decide the outcome involving performance.
What is a good and affordable server hosting company? Good and affordable - two things that we can only wish for. Personally, I used to use Nitrous Networks, as they offer very good quality services, but for a quite steep price, considering the competitors on the market. Now I'd say go with HostHorde, as you can get unlimited RAM for £15 a month, which from other server hosts, could range up to £50 a month.
All in all, I wish you luck with your server, and if you need any help, message me on skype at 'hellcraftluke'.
Luke.