Dawn of War Professional, also known as DoWpro, is a Mod for Dawn of War. The mod centers about bringing a gameplay-experience similar to Vanilla Dawn of War (pre-Winter Assault), while implementing cut or missing content.
Dawn of War Professional focuses primarily on bringing the "feel" of the Vanilla Dawn of War to the later expansions - with the slower pacing and more procedural upgrades. Much of the original games' cut content was added in the mod, as well as in general, more toys for each faction to play as - the objective being that the player is generally given more choices as to the composition of their armies.
Dawn of War Professional also implements a number of cosmetic and minor improvements, in an effort to help keep some of the units closer to fluff. As an example, the World Eaters-exclusive Khorne Berserker in Vanilla is replaced with a Black Legion-style Berserker that includes team colors to make it look like a Khornate unit that happens to be in the warband. Other changes were made throughout the various factions' tech trees and on a unit-by-unit basis to re-balance the game mechanics more in-line with Vanilla as well (such as removing the fire on the move accuracy "fix" implemented in Winter Assault). This, however, lead to a series of "pancake-style" tech-upgrades ala 90's strategy-games, e.g. StarCraft. Some factions are changed more than others; Necrons use a unique mechanic where they need to expand their area of control and actually gain resources in an effort to actually force them to take and hold territory and play more like the rest of the factions, as opposed to how they originally played.
DoWpro also implements new content from certain other mods (used with permission), most notably the Stronghold Assault mod.
As a mod that focuses on expanding the Vanilla experience, Dawn of War Professional has a number of issues that make sure it most-assuredly isn't for everyone, as well as a number of gameplay issues and bugs all its own.
Dawn of War Professional has a much slower game pace, with a much larger focus on expanding and holding territory as opposed to the much more aggressive fast-tech-and-fight gameplay of Winter Assault, Dark Crusade, and Soulstorm. This leads to matches being significantly longer.
Even though the game is slower in pace, it's also much less forgiving due to focusing on the Vanilla paradigm. Units in general are much more fragile than they are in the originals, especially when it comes to mid-game vehicles and weapons versus the biggest units of the endgame. This ironically makes for a game that while certainly much more varied than Vanilla, also tends to culminate in much more protracted battles over the course of a given match, and can quickly boil down to drawn-out attrition fights of "who makes the first mistake" in the worst cases (Orks vs Imperial Guard on a huge map for example), or games that were technically won halfway through the match.
It also fundamentally changes how the factions added in the expansions play, and not always for the better. Some factions, e.g. the Imperial Guard and Tau, mostly gets new stuff, whereas others, e.g. The Necrons and Sisters of Battle, are changed so as to play like an entirely new faciton, as opposed to how their originally played. Many units are also very different from how they're handled in the basic games, and the mod removes flyers.
Dawn of War Professional also changes a number of mechanics. Stealth, for example in Professional is much different from Vanilla "invisible only if not shooting" or Dark Crusade "Always invisible all the time". Instead, Dowpro has three specific types of Stealth depending on unit; "invisible when in cover", "invisible when not attacking", and "invisible all the time". Even some units which keep fundamentally the same role they had in the original game may be changed in implementation, to the point where their use is fundamentally different in practice. The return of Hard Counters also means that there are comparatively few generalist units in the game, with the focus being on units doing a specific job a certain way instead.
However, your mileage may vary; not all problems are simply a matter of "getting used to the way the mod plays", or "being tactical enough"; it's highly possible to just plain not enjoy the mod for what it is, no matter how well one learns and understands it and how it works. It makes the game as such play differently (the base game plus expansions could be compared to e.g. Starcraft in it's extremely aggressive and strategic play, whereas the mod is more slow and tactical), and it's not simply a matter of "being new or inexperienced". Try it out; some like it, some don't.
Those who want an experience closer to Vanilla will likely enjoy the mod, but those who are fans of how existing factions or mechanics already work or feel (e.g. the uniqueness of the Necrons, which the mod removes) certainly won't enjoy it.
Further, it has a number of unique bugs (some of which might be blamed on the core game-engine), some of which are only avoidable by being aware of them (i.e. have to actively be played around by the player).
- http://www.moddb.com/mods/dowpro - Mod DB page.