These are what we call the 'YMMV items.' Things that some people find in this work. We call them 'your mileage might vary' because not everyone sees these things in the same way. This starts discussions in the trope lists, a thing we don't want. Please use the discussion page if you'd like to discuss any of these items.
Character Tiers: Expect a fair number of posts on any Diplomacy forum to discuss about which power is the best. Statistically, Russia is mostly likely to win, though that trend has also been extended to Turkey and France. Similarly, Italy is widely considered to be the worst power, as it is the worst situation at the start of the game: it has limited territory to expand, being sandwiched between France and Austria-Hungary.
In least-to-best order: Italy, Austria, Germany, Turkey/England, France, Russia. (Source: statistics at the Diplomacy Archive, which are somewhat out of date but still more or less accepted as standard.)
This is VERY serious YMMV territory. Many, many people believe that France is the best power. The only things widely (though by no means universally) acknowledged are that Italy is the weakest and that being a central power (Austria or Germany) isn't great.
Similarly, there is a large school of thought that hates playing as Turkey. While the corner position makes it the easiest to defend, it also has a very hard getting started offensively and while it takes some time, a dedicated alliance can pick it apart.
While Italy starts weak geographically, it has a big advantage diplomatically: it's involved in neither the western (England-Germany-France) nor the eastern (Austria-Turkey-Russia) triangle; as such, Italy is an attractive ally for everyone, but not a prime target for early aggression. Whether this advantage makes up for the positional disadvantage is a matter of opinion.
Crowning Moment of Awesome: YMMV, but if you like this kind of game, these are gonna come up A LOT when you pull of a particularly notable backstab or surprise attack.
Crowning Moment of Funny: Many (the most common would be a fleet ending up in very odd place, as an Italian fleet in St. Pete, which has happened), but the best of them all is probably The Coast of Moscow, as recorded by the game's creator. (If you're confused, here's a map◊.)
Manipulative Bastard: Most people who have actually managed solo wins can also justifiably claim to be one of these.
This troper experienced this game in European History class where each country was played by a team so everyone would be involved. The team that did best in all the games we played paired a Manipulative Bastard with a trust-worthy and honest partner. Essentially, the bastard (this troper) plotted out all our devious plans while the trusted partner convinced potential allies to support our moves. After a few matches, the only people who would ally with us at all were those who could not geographically be reached (as Turkey we convinced England to ally with us and managed to carve up Russia and Austria and start moving on the rest of the continent before people had even realized what was going on)